[v4,2/2] mailbox: Add Tegra HSP driver

From: Thierry Reding <treding@nvidia.com>

This driver exposes a mailbox interface for interprocessor communication
using the Hardware Synchronization Primitives (HSP) module's doorbell
mechanism. There are multiple HSP instances and they provide additional
features such as shared mailboxes, shared and arbitrated semaphores.

A driver for a remote processor can use the mailbox client provided by
the HSP driver and build an IPC protocol on top of this synchronization
mechanism.

Based on work by Joseph Lo <josephl@nvidia.com>.

Signed-off-by: Thierry Reding <treding@nvidia.com>
---
Changes in v4:
- check for doorbell ring before enabling new master
- add missing space in Kconfig description
- remove unnecessary barriers
- simplify doorbell lookup
- remove debug leftovers

Changes in v3:
- use a more object oriented design
- fix some locking issues

 drivers/mailbox/Kconfig     |   9 +
 drivers/mailbox/Makefile    |   2 +
 drivers/mailbox/tegra-hsp.c | 542 ++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 553 insertions(+)
 create mode 100644 drivers/mailbox/tegra-hsp.c

On Tue, Nov 15, 2016 at 9:18 PM, Thierry Reding
<thierry.reding@gmail.com> wrote:

....
> +
> +struct tegra_hsp_channel;
> +struct tegra_hsp;
> +
> +struct tegra_hsp_channel_ops {
> +       int (*send_data)(struct tegra_hsp_channel *channel, void *data);
> +       int (*startup)(struct tegra_hsp_channel *channel);
> +       void (*shutdown)(struct tegra_hsp_channel *channel);
> +       bool (*last_tx_done)(struct tegra_hsp_channel *channel);
> +};
> +
> +struct tegra_hsp_channel {
> +       struct tegra_hsp *hsp;
> +       const struct tegra_hsp_channel_ops *ops;
> +       struct mbox_chan *chan;
> +       void __iomem *regs;
> +};
> +
> +static struct tegra_hsp_channel *to_tegra_hsp_channel(struct mbox_chan *chan)
> +{
> +       return chan->con_priv;
> +}
> +
It seems
       channel = to_tegra_hsp_channel(chan);
is no simpler ritual than
       channel = chan->con_priv;   ?

> +struct tegra_hsp_doorbell {
> +       struct tegra_hsp_channel channel;
> +       struct list_head list;
> +       const char *name;
> +       unsigned int master;
> +       unsigned int index;
> +};
> +
> +static struct tegra_hsp_doorbell *
> +to_tegra_hsp_doorbell(struct tegra_hsp_channel *channel)
> +{
> +       if (!channel)
> +               return NULL;
> +
> +       return container_of(channel, struct tegra_hsp_doorbell, channel);
> +}
> +
But you don't check for NULL returned, before dereferencing the pointer 'db'

> +struct tegra_hsp_db_map {
> +       const char *name;
> +       unsigned int master;
> +       unsigned int index;
> +};
> +
> +struct tegra_hsp_soc {
> +       const struct tegra_hsp_db_map *map;
> +};
> +
> +struct tegra_hsp {
> +       const struct tegra_hsp_soc *soc;
> +       struct mbox_controller mbox;
> +       void __iomem *regs;
> +       unsigned int irq;
> +       unsigned int num_sm;
> +       unsigned int num_as;
> +       unsigned int num_ss;
> +       unsigned int num_db;
> +       unsigned int num_si;
> +       spinlock_t lock;
> +
> +       struct list_head doorbells;
> +};
> +
> +static inline struct tegra_hsp *
> +to_tegra_hsp(struct mbox_controller *mbox)
> +{
> +       return container_of(mbox, struct tegra_hsp, mbox);
> +}
> +
> +static inline u32 tegra_hsp_readl(struct tegra_hsp *hsp, unsigned int offset)
> +{
> +       return readl(hsp->regs + offset);
> +}
> +
> +static inline void tegra_hsp_writel(struct tegra_hsp *hsp, u32 value,
> +                                   unsigned int offset)
> +{
> +       writel(value, hsp->regs + offset);
> +}
> +
> +static inline u32 tegra_hsp_channel_readl(struct tegra_hsp_channel *channel,
> +                                         unsigned int offset)
> +{
> +       return readl(channel->regs + offset);
> +}
> +
> +static inline void tegra_hsp_channel_writel(struct tegra_hsp_channel *channel,
> +                                           u32 value, unsigned int offset)
> +{
> +       writel(value, channel->regs + offset);
> +}
> +
> +static bool tegra_hsp_doorbell_can_ring(struct tegra_hsp_doorbell *db)
> +{
> +       u32 value;
> +
> +       value = tegra_hsp_channel_readl(&db->channel, HSP_DB_ENABLE);
> +
> +       return (value & BIT(TEGRA_HSP_DB_MASTER_CCPLEX)) != 0;
> +}
> +
> +static struct tegra_hsp_doorbell *
> +__tegra_hsp_doorbell_get(struct tegra_hsp *hsp, unsigned int master)
> +{
> +       struct tegra_hsp_doorbell *entry;
> +
> +       list_for_each_entry(entry, &hsp->doorbells, list)
> +               if (entry->master == master)
> +                       return entry;
> +
> +       return NULL;
> +}
> +
> +static struct tegra_hsp_doorbell *
> +tegra_hsp_doorbell_get(struct tegra_hsp *hsp, unsigned int master)
> +{
> +       struct tegra_hsp_doorbell *db;
> +       unsigned long flags;
> +
> +       spin_lock_irqsave(&hsp->lock, flags);
> +       db = __tegra_hsp_doorbell_get(hsp, master);
> +       spin_unlock_irqrestore(&hsp->lock, flags);
> +
> +       return db;
> +}
> +
.....

> +
> +static int tegra_hsp_doorbell_send_data(struct tegra_hsp_channel *channel,
> +                                       void *data)
> +{
> +       tegra_hsp_channel_writel(channel, 1, HSP_DB_TRIGGER);
> +
> +       return 0;
> +}
> +
> +static int tegra_hsp_doorbell_startup(struct tegra_hsp_channel *channel)
> +{
> +       struct tegra_hsp_doorbell *db = to_tegra_hsp_doorbell(channel);
> +       struct tegra_hsp *hsp = channel->hsp;
> +       struct tegra_hsp_doorbell *ccplex;
> +       unsigned long flags;
> +       u32 value;
> +
> +       if (db->master >= hsp->mbox.num_chans) {
> +               dev_err(hsp->mbox.dev,
> +                       "invalid master ID %u for HSP channel\n",
> +                       db->master);
> +               return -EINVAL;
> +       }
> +
> +       ccplex = tegra_hsp_doorbell_get(hsp, TEGRA_HSP_DB_MASTER_CCPLEX);
> +       if (!ccplex)
> +               return -ENODEV;
> +
> +       if (!tegra_hsp_doorbell_can_ring(db))
> +               return -ENODEV;
> +
> +       spin_lock_irqsave(&hsp->lock, flags);
> +
> +       value = tegra_hsp_channel_readl(&ccplex->channel, HSP_DB_ENABLE);
> +       value |= BIT(db->master);
> +       tegra_hsp_channel_writel(&ccplex->channel, value, HSP_DB_ENABLE);
> +
> +       spin_unlock_irqrestore(&hsp->lock, flags);
> +
> +       return 0;
> +}
> +
> +static void tegra_hsp_doorbell_shutdown(struct tegra_hsp_channel *channel)
> +{
> +       struct tegra_hsp_doorbell *db = to_tegra_hsp_doorbell(channel);
> +       struct tegra_hsp *hsp = channel->hsp;
> +       struct tegra_hsp_doorbell *ccplex;
> +       unsigned long flags;
> +       u32 value;
> +
> +       ccplex = tegra_hsp_doorbell_get(hsp, TEGRA_HSP_DB_MASTER_CCPLEX);
> +       if (!ccplex)
> +               return;
> +
> +       spin_lock_irqsave(&hsp->lock, flags);
> +
> +       value = tegra_hsp_channel_readl(&ccplex->channel, HSP_DB_ENABLE);
> +       value &= ~BIT(db->master);
> +       tegra_hsp_channel_writel(&ccplex->channel, value, HSP_DB_ENABLE);
> +
> +       spin_unlock_irqrestore(&hsp->lock, flags);
> +}
> +
> +static bool tegra_hsp_doorbell_last_tx_done(struct tegra_hsp_channel *channel)
> +{
> +       return true;
> +}
Just curious, is the IPC done instantly after writing HSP_DB_TRIGGER
bit? Usually there is at least some bit that stays (un)set as a 'busy
flag'.

> +
> +static const struct tegra_hsp_channel_ops tegra_hsp_doorbell_ops = {
> +       .send_data = tegra_hsp_doorbell_send_data,
> +       .startup = tegra_hsp_doorbell_startup,
> +       .shutdown = tegra_hsp_doorbell_shutdown,
> +       .last_tx_done = tegra_hsp_doorbell_last_tx_done,
> +};
> +
....

> +static int tegra_hsp_send_data(struct mbox_chan *chan, void *data)
> +{
> +       struct tegra_hsp_channel *channel = to_tegra_hsp_channel(chan);
> +
> +       return channel->ops->send_data(channel, data);
> +}
> +
> +static int tegra_hsp_startup(struct mbox_chan *chan)
> +{
> +       struct tegra_hsp_channel *channel = to_tegra_hsp_channel(chan);
> +
> +       return channel->ops->startup(channel);
> +}
> +
> +static void tegra_hsp_shutdown(struct mbox_chan *chan)
> +{
> +       struct tegra_hsp_channel *channel = to_tegra_hsp_channel(chan);
> +
> +       return channel->ops->shutdown(channel);
> +}
> +
> +static bool tegra_hsp_last_tx_done(struct mbox_chan *chan)
> +{
> +       struct tegra_hsp_channel *channel = to_tegra_hsp_channel(chan);
> +
> +       return channel->ops->last_tx_done(channel);
> +}
> +
> +static const struct mbox_chan_ops tegra_hsp_ops = {
> +       .send_data = tegra_hsp_send_data,
> +       .startup = tegra_hsp_startup,
> +       .shutdown = tegra_hsp_shutdown,
> +       .last_tx_done = tegra_hsp_last_tx_done,
> +};
> +
These 4 above seem overkill. Why not directly use tegra_hsp_doorbell_xxx() ?

Cheers!
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On Wed, Nov 16, 2016 at 10:58:07AM +0530, Jassi Brar wrote:
> On Tue, Nov 15, 2016 at 9:18 PM, Thierry Reding
> <thierry.reding@gmail.com> wrote:
> 
> ....
> > +
> > +struct tegra_hsp_channel;
> > +struct tegra_hsp;
> > +
> > +struct tegra_hsp_channel_ops {
> > +       int (*send_data)(struct tegra_hsp_channel *channel, void *data);
> > +       int (*startup)(struct tegra_hsp_channel *channel);
> > +       void (*shutdown)(struct tegra_hsp_channel *channel);
> > +       bool (*last_tx_done)(struct tegra_hsp_channel *channel);
> > +};
> > +
> > +struct tegra_hsp_channel {
> > +       struct tegra_hsp *hsp;
> > +       const struct tegra_hsp_channel_ops *ops;
> > +       struct mbox_chan *chan;
> > +       void __iomem *regs;
> > +};
> > +
> > +static struct tegra_hsp_channel *to_tegra_hsp_channel(struct mbox_chan *chan)
> > +{
> > +       return chan->con_priv;
> > +}
> > +
> It seems
>        channel = to_tegra_hsp_channel(chan);
> is no simpler ritual than
>        channel = chan->con_priv;   ?

Yes, that's true. I've dropped the to_tegra_hsp_channel() inline in
favour of using the chan->con_priv directly.

> > +struct tegra_hsp_doorbell {
> > +       struct tegra_hsp_channel channel;
> > +       struct list_head list;
> > +       const char *name;
> > +       unsigned int master;
> > +       unsigned int index;
> > +};
> > +
> > +static struct tegra_hsp_doorbell *
> > +to_tegra_hsp_doorbell(struct tegra_hsp_channel *channel)
> > +{
> > +       if (!channel)
> > +               return NULL;
> > +
> > +       return container_of(channel, struct tegra_hsp_doorbell, channel);
> > +}
> > +
> But you don't check for NULL returned, before dereferencing the pointer 'db'

In all the call sites where this is used the channel is guaranteed not
to be NULL, hence no checking is necessary. However the function here
could potentially be used in other cases where no such guarantees can
be given and checking the !channel above is merely there to avoid
casting to a non-NULL pointer from a NULL pointer.

I've run occasionally into this issue because container_of() will simply
perform arithmetic on the pointer given, so passing channel as NULL
would convert to some very large pointer that can no longer be easily
discerned from an invalid pointer.

So this is primarily a safety feature, and one that I'd prefer to keep
just to avoid running into issues down the road when the function gets
used under different circumstances.

> > +static bool tegra_hsp_doorbell_last_tx_done(struct tegra_hsp_channel *channel)
> > +{
> > +       return true;
> > +}
> Just curious, is the IPC done instantly after writing HSP_DB_TRIGGER
> bit? Usually there is at least some bit that stays (un)set as a 'busy
> flag'.

I don't think there's a bit like that for doorbells. The way that these
doorbells are used is in combination with a shared memory IPC protocol.
Two processors will communicate by writing to and reading from what is
essentially a ring buffer in shared memory. The doorbells are merely a
means of communicating their peer that a new entry is available in the
shared memory.

> > +static const struct tegra_hsp_channel_ops tegra_hsp_doorbell_ops = {
> > +       .send_data = tegra_hsp_doorbell_send_data,
> > +       .startup = tegra_hsp_doorbell_startup,
> > +       .shutdown = tegra_hsp_doorbell_shutdown,
> > +       .last_tx_done = tegra_hsp_doorbell_last_tx_done,
> > +};
> > +
> ....
> 
> > +static int tegra_hsp_send_data(struct mbox_chan *chan, void *data)
> > +{
> > +       struct tegra_hsp_channel *channel = to_tegra_hsp_channel(chan);
> > +
> > +       return channel->ops->send_data(channel, data);
> > +}
> > +
> > +static int tegra_hsp_startup(struct mbox_chan *chan)
> > +{
> > +       struct tegra_hsp_channel *channel = to_tegra_hsp_channel(chan);
> > +
> > +       return channel->ops->startup(channel);
> > +}
> > +
> > +static void tegra_hsp_shutdown(struct mbox_chan *chan)
> > +{
> > +       struct tegra_hsp_channel *channel = to_tegra_hsp_channel(chan);
> > +
> > +       return channel->ops->shutdown(channel);
> > +}
> > +
> > +static bool tegra_hsp_last_tx_done(struct mbox_chan *chan)
> > +{
> > +       struct tegra_hsp_channel *channel = to_tegra_hsp_channel(chan);
> > +
> > +       return channel->ops->last_tx_done(channel);
> > +}
> > +
> > +static const struct mbox_chan_ops tegra_hsp_ops = {
> > +       .send_data = tegra_hsp_send_data,
> > +       .startup = tegra_hsp_startup,
> > +       .shutdown = tegra_hsp_shutdown,
> > +       .last_tx_done = tegra_hsp_last_tx_done,
> > +};
> > +
> These 4 above seem overkill. Why not directly use tegra_hsp_doorbell_xxx() ?

This is in preparation for supporting the other synchronization
primitives that the HSP IP block exposes. Some of them use different
programming and semantics, hence why we want to have this second level
of abstraction. It will allow us to share some of the code between the
different primitives once their implementations are added.

If you feel really strongly about it, I suppose I could eliminate it,
but I suspect that we'd want to add them back in after support for the
other primitives is added.

Thanks,
Thierry

On Wed, Nov 16, 2016 at 8:38 PM, Thierry Reding
<thierry.reding@gmail.com> wrote:
> On Wed, Nov 16, 2016 at 10:58:07AM +0530, Jassi Brar wrote:
>> On Tue, Nov 15, 2016 at 9:18 PM, Thierry Reding
>> <thierry.reding@gmail.com> wrote:
>>
>> ....
>> > +
>> > +struct tegra_hsp_channel;
>> > +struct tegra_hsp;
>> > +
>> > +struct tegra_hsp_channel_ops {
>> > +       int (*send_data)(struct tegra_hsp_channel *channel, void *data);
>> > +       int (*startup)(struct tegra_hsp_channel *channel);
>> > +       void (*shutdown)(struct tegra_hsp_channel *channel);
>> > +       bool (*last_tx_done)(struct tegra_hsp_channel *channel);
>> > +};
>> > +
>> > +struct tegra_hsp_channel {
>> > +       struct tegra_hsp *hsp;
>> > +       const struct tegra_hsp_channel_ops *ops;
>> > +       struct mbox_chan *chan;
>> > +       void __iomem *regs;
>> > +};
>> > +
>> > +static struct tegra_hsp_channel *to_tegra_hsp_channel(struct mbox_chan *chan)
>> > +{
>> > +       return chan->con_priv;
>> > +}
>> > +
>> It seems
>>        channel = to_tegra_hsp_channel(chan);
>> is no simpler ritual than
>>        channel = chan->con_priv;   ?
>
> Yes, that's true. I've dropped the to_tegra_hsp_channel() inline in
> favour of using the chan->con_priv directly.
>
>> > +struct tegra_hsp_doorbell {
>> > +       struct tegra_hsp_channel channel;
>> > +       struct list_head list;
>> > +       const char *name;
>> > +       unsigned int master;
>> > +       unsigned int index;
>> > +};
>> > +
>> > +static struct tegra_hsp_doorbell *
>> > +to_tegra_hsp_doorbell(struct tegra_hsp_channel *channel)
>> > +{
>> > +       if (!channel)
>> > +               return NULL;
>> > +
>> > +       return container_of(channel, struct tegra_hsp_doorbell, channel);
>> > +}
>> > +
>> But you don't check for NULL returned, before dereferencing the pointer 'db'
>
> In all the call sites where this is used the channel is guaranteed not
> to be NULL, hence no checking is necessary. However the function here
> could potentially be used in other cases where no such guarantees can
> be given and checking the !channel above is merely there to avoid
> casting to a non-NULL pointer from a NULL pointer.
>
> I've run occasionally into this issue because container_of() will simply
> perform arithmetic on the pointer given, so passing channel as NULL
> would convert to some very large pointer that can no longer be easily
> discerned from an invalid pointer.
>
> So this is primarily a safety feature, and one that I'd prefer to keep
> just to avoid running into issues down the road when the function gets
> used under different circumstances.
>
>> > +static bool tegra_hsp_doorbell_last_tx_done(struct tegra_hsp_channel *channel)
>> > +{
>> > +       return true;
>> > +}
>> Just curious, is the IPC done instantly after writing HSP_DB_TRIGGER
>> bit? Usually there is at least some bit that stays (un)set as a 'busy
>> flag'.
>
> I don't think there's a bit like that for doorbells. The way that these
> doorbells are used is in combination with a shared memory IPC protocol.
> Two processors will communicate by writing to and reading from what is
> essentially a ring buffer in shared memory. The doorbells are merely a
> means of communicating their peer that a new entry is available in the
> shared memory.
>
For such protocols, we have the TXDONE_BY_ACK. I assume your client
drivers will drive the state-machine. Otherwise, you risk overrunning
the ring-buffer in SHM, but not caring if the first filled buffer was
actually consumed by the remote (just like ALSA ring buffer).

>> > +static const struct tegra_hsp_channel_ops tegra_hsp_doorbell_ops = {
>> > +       .send_data = tegra_hsp_doorbell_send_data,
>> > +       .startup = tegra_hsp_doorbell_startup,
>> > +       .shutdown = tegra_hsp_doorbell_shutdown,
>> > +       .last_tx_done = tegra_hsp_doorbell_last_tx_done,
>> > +};
>> > +
>> ....
>>
>> > +static int tegra_hsp_send_data(struct mbox_chan *chan, void *data)
>> > +{
>> > +       struct tegra_hsp_channel *channel = to_tegra_hsp_channel(chan);
>> > +
>> > +       return channel->ops->send_data(channel, data);
>> > +}
>> > +
>> > +static int tegra_hsp_startup(struct mbox_chan *chan)
>> > +{
>> > +       struct tegra_hsp_channel *channel = to_tegra_hsp_channel(chan);
>> > +
>> > +       return channel->ops->startup(channel);
>> > +}
>> > +
>> > +static void tegra_hsp_shutdown(struct mbox_chan *chan)
>> > +{
>> > +       struct tegra_hsp_channel *channel = to_tegra_hsp_channel(chan);
>> > +
>> > +       return channel->ops->shutdown(channel);
>> > +}
>> > +
>> > +static bool tegra_hsp_last_tx_done(struct mbox_chan *chan)
>> > +{
>> > +       struct tegra_hsp_channel *channel = to_tegra_hsp_channel(chan);
>> > +
>> > +       return channel->ops->last_tx_done(channel);
>> > +}
>> > +
>> > +static const struct mbox_chan_ops tegra_hsp_ops = {
>> > +       .send_data = tegra_hsp_send_data,
>> > +       .startup = tegra_hsp_startup,
>> > +       .shutdown = tegra_hsp_shutdown,
>> > +       .last_tx_done = tegra_hsp_last_tx_done,
>> > +};
>> > +
>> These 4 above seem overkill. Why not directly use tegra_hsp_doorbell_xxx() ?
>
> This is in preparation for supporting the other synchronization
> primitives that the HSP IP block exposes. Some of them use different
> programming and semantics, hence why we want to have this second level
> of abstraction. It will allow us to share some of the code between the
> different primitives once their implementations are added.
>
OK, but until then this, and the above NULL check, will look silly.
Usually we add only necessary code at any time.

Thanks.
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On Wed, Nov 16, 2016 at 09:00:19PM +0530, Jassi Brar wrote:
> On Wed, Nov 16, 2016 at 8:38 PM, Thierry Reding
> <thierry.reding@gmail.com> wrote:
> > On Wed, Nov 16, 2016 at 10:58:07AM +0530, Jassi Brar wrote:
> >> On Tue, Nov 15, 2016 at 9:18 PM, Thierry Reding
> >> <thierry.reding@gmail.com> wrote:
> >>
> >> ....
> >> > +
> >> > +struct tegra_hsp_channel;
> >> > +struct tegra_hsp;
> >> > +
> >> > +struct tegra_hsp_channel_ops {
> >> > +       int (*send_data)(struct tegra_hsp_channel *channel, void *data);
> >> > +       int (*startup)(struct tegra_hsp_channel *channel);
> >> > +       void (*shutdown)(struct tegra_hsp_channel *channel);
> >> > +       bool (*last_tx_done)(struct tegra_hsp_channel *channel);
> >> > +};
> >> > +
> >> > +struct tegra_hsp_channel {
> >> > +       struct tegra_hsp *hsp;
> >> > +       const struct tegra_hsp_channel_ops *ops;
> >> > +       struct mbox_chan *chan;
> >> > +       void __iomem *regs;
> >> > +};
> >> > +
> >> > +static struct tegra_hsp_channel *to_tegra_hsp_channel(struct mbox_chan *chan)
> >> > +{
> >> > +       return chan->con_priv;
> >> > +}
> >> > +
> >> It seems
> >>        channel = to_tegra_hsp_channel(chan);
> >> is no simpler ritual than
> >>        channel = chan->con_priv;   ?
> >
> > Yes, that's true. I've dropped the to_tegra_hsp_channel() inline in
> > favour of using the chan->con_priv directly.
> >
> >> > +struct tegra_hsp_doorbell {
> >> > +       struct tegra_hsp_channel channel;
> >> > +       struct list_head list;
> >> > +       const char *name;
> >> > +       unsigned int master;
> >> > +       unsigned int index;
> >> > +};
> >> > +
> >> > +static struct tegra_hsp_doorbell *
> >> > +to_tegra_hsp_doorbell(struct tegra_hsp_channel *channel)
> >> > +{
> >> > +       if (!channel)
> >> > +               return NULL;
> >> > +
> >> > +       return container_of(channel, struct tegra_hsp_doorbell, channel);
> >> > +}
> >> > +
> >> But you don't check for NULL returned, before dereferencing the pointer 'db'
> >
> > In all the call sites where this is used the channel is guaranteed not
> > to be NULL, hence no checking is necessary. However the function here
> > could potentially be used in other cases where no such guarantees can
> > be given and checking the !channel above is merely there to avoid
> > casting to a non-NULL pointer from a NULL pointer.
> >
> > I've run occasionally into this issue because container_of() will simply
> > perform arithmetic on the pointer given, so passing channel as NULL
> > would convert to some very large pointer that can no longer be easily
> > discerned from an invalid pointer.
> >
> > So this is primarily a safety feature, and one that I'd prefer to keep
> > just to avoid running into issues down the road when the function gets
> > used under different circumstances.
> >
> >> > +static bool tegra_hsp_doorbell_last_tx_done(struct tegra_hsp_channel *channel)
> >> > +{
> >> > +       return true;
> >> > +}
> >> Just curious, is the IPC done instantly after writing HSP_DB_TRIGGER
> >> bit? Usually there is at least some bit that stays (un)set as a 'busy
> >> flag'.
> >
> > I don't think there's a bit like that for doorbells. The way that these
> > doorbells are used is in combination with a shared memory IPC protocol.
> > Two processors will communicate by writing to and reading from what is
> > essentially a ring buffer in shared memory. The doorbells are merely a
> > means of communicating their peer that a new entry is available in the
> > shared memory.
> >
> For such protocols, we have the TXDONE_BY_ACK. I assume your client
> drivers will drive the state-machine. Otherwise, you risk overrunning
> the ring-buffer in SHM, but not caring if the first filled buffer was
> actually consumed by the remote (just like ALSA ring buffer).

I did some digging and it turns out that the driver is already using
TXDONE_BY_ACK (it sets .txdone_irq = false and .txdone_poll = false)
and indeed users are driving the TX state machine by calling the
mbox_client_txdone() function where appropriate.

Given the above the .last_tx_done() callback is completely unused so
I've just dropped it.

> >> > +static const struct tegra_hsp_channel_ops tegra_hsp_doorbell_ops = {
> >> > +       .send_data = tegra_hsp_doorbell_send_data,
> >> > +       .startup = tegra_hsp_doorbell_startup,
> >> > +       .shutdown = tegra_hsp_doorbell_shutdown,
> >> > +       .last_tx_done = tegra_hsp_doorbell_last_tx_done,
> >> > +};
> >> > +
> >> ....
> >>
> >> > +static int tegra_hsp_send_data(struct mbox_chan *chan, void *data)
> >> > +{
> >> > +       struct tegra_hsp_channel *channel = to_tegra_hsp_channel(chan);
> >> > +
> >> > +       return channel->ops->send_data(channel, data);
> >> > +}
> >> > +
> >> > +static int tegra_hsp_startup(struct mbox_chan *chan)
> >> > +{
> >> > +       struct tegra_hsp_channel *channel = to_tegra_hsp_channel(chan);
> >> > +
> >> > +       return channel->ops->startup(channel);
> >> > +}
> >> > +
> >> > +static void tegra_hsp_shutdown(struct mbox_chan *chan)
> >> > +{
> >> > +       struct tegra_hsp_channel *channel = to_tegra_hsp_channel(chan);
> >> > +
> >> > +       return channel->ops->shutdown(channel);
> >> > +}
> >> > +
> >> > +static bool tegra_hsp_last_tx_done(struct mbox_chan *chan)
> >> > +{
> >> > +       struct tegra_hsp_channel *channel = to_tegra_hsp_channel(chan);
> >> > +
> >> > +       return channel->ops->last_tx_done(channel);
> >> > +}
> >> > +
> >> > +static const struct mbox_chan_ops tegra_hsp_ops = {
> >> > +       .send_data = tegra_hsp_send_data,
> >> > +       .startup = tegra_hsp_startup,
> >> > +       .shutdown = tegra_hsp_shutdown,
> >> > +       .last_tx_done = tegra_hsp_last_tx_done,
> >> > +};
> >> > +
> >> These 4 above seem overkill. Why not directly use tegra_hsp_doorbell_xxx() ?
> >
> > This is in preparation for supporting the other synchronization
> > primitives that the HSP IP block exposes. Some of them use different
> > programming and semantics, hence why we want to have this second level
> > of abstraction. It will allow us to share some of the code between the
> > different primitives once their implementations are added.
> >
> OK, but until then this, and the above NULL check, will look silly.
> Usually we add only necessary code at any time.

I've removed the additional level of indirection, we can always add this
back when/if required. As a side-effect the to_tegra_hsp_doorbell() goes
away because it is unused, thereby removing your last concern as well.

I just sent out a v5 that should address all of your comments.

Thanks,
Thierry