| Message ID | 1278653389-12019-1-git-send-email-sudhakar.raj@ti.com |
|---|---|
| State | New, archived |
| Headers | show |
On Fri, 9 Jul 2010 10:59:49 +0530 Sudhakar Rajashekhara <sudhakar.raj@ti.com> wrote: > + > + /* > + * ECC_STATE field reads 0x3 (Error correction complete) immediately > + * after setting the 4BITECC_ADD_CALC_START bit. So if you immediately > + * begin trying to poll for the state, you may fall right out of your > + * loop without any of the correction calculations having taken place. > + * The recommendation from the hardware team is to wait till ECC_STATE > + * reads less than 4, which means ECC HW has entered correction state. > + */ > + do { > + ecc_state = (davinci_nand_readl(info, > + NANDFSR_OFFSET) >> 8) & 0x0f; > + cpu_relax(); > + } while ((ecc_state < 4) && time_before(jiffies, timeo)); An up-to-100-milliseond busy wait is pretty bad. For how long do you expect this to spin in practice?
On Sat, Jul 10, 2010 at 04:09:32, Andrew Morton wrote: > On Fri, 9 Jul 2010 10:59:49 +0530 > Sudhakar Rajashekhara <sudhakar.raj@ti.com> wrote: > > > + > > + /* > > + * ECC_STATE field reads 0x3 (Error correction complete) immediately > > + * after setting the 4BITECC_ADD_CALC_START bit. So if you immediately > > + * begin trying to poll for the state, you may fall right out of your > > + * loop without any of the correction calculations having taken place. > > + * The recommendation from the hardware team is to wait till ECC_STATE > > + * reads less than 4, which means ECC HW has entered correction state. > > + */ > > + do { > > + ecc_state = (davinci_nand_readl(info, > > + NANDFSR_OFFSET) >> 8) & 0x0f; > > + cpu_relax(); > > + } while ((ecc_state < 4) && time_before(jiffies, timeo)); > > An up-to-100-milliseond busy wait is pretty bad. For how long do you > expect this to spin in practice? On the hardware, I have never seen this taking 100 msec to come out of the loop. I'll check with the hardware folks on the maximum time to wait for, before the ECC engine is ready. Thanks, Sudhakar
Hi, On Mon, Jul 12, 2010 at 11:58:18, Sudhakar Rajashekhara wrote: > On Sat, Jul 10, 2010 at 04:09:32, Andrew Morton wrote: > > On Fri, 9 Jul 2010 10:59:49 +0530 > > Sudhakar Rajashekhara <sudhakar.raj@ti.com> wrote: > > > > > + > > > + /* > > > + * ECC_STATE field reads 0x3 (Error correction complete) immediately > > > + * after setting the 4BITECC_ADD_CALC_START bit. So if you immediately > > > + * begin trying to poll for the state, you may fall right out of your > > > + * loop without any of the correction calculations having taken place. > > > + * The recommendation from the hardware team is to wait till ECC_STATE > > > + * reads less than 4, which means ECC HW has entered correction state. > > > + */ > > > + do { > > > + ecc_state = (davinci_nand_readl(info, > > > + NANDFSR_OFFSET) >> 8) & 0x0f; > > > + cpu_relax(); > > > + } while ((ecc_state < 4) && time_before(jiffies, timeo)); > > > > An up-to-100-milliseond busy wait is pretty bad. For how long do you > > expect this to spin in practice? > > On the hardware, I have never seen this taking 100 msec to come out of > the loop. I'll check with the hardware folks on the maximum time to wait > for, before the ECC engine is ready. I checked this with the hardware team but no one is sure about the exact time one should wait before the ECC engine becomes ready. But everyone is of the opinion that 100 loop cycles should be enough. To be on the safer side, I'll be changing the timeout to 10 milliseconds in the next version of this patch. Thanks, Sudhakar
On Tue, 13 Jul 2010 15:02:59 +0530 "Sudhakar Rajashekhara" <sudhakar.raj@ti.com> wrote: > Hi, > > On Mon, Jul 12, 2010 at 11:58:18, Sudhakar Rajashekhara wrote: > > On Sat, Jul 10, 2010 at 04:09:32, Andrew Morton wrote: > > > On Fri, 9 Jul 2010 10:59:49 +0530 > > > Sudhakar Rajashekhara <sudhakar.raj@ti.com> wrote: > > > > > > > + > > > > + /* > > > > + * ECC_STATE field reads 0x3 (Error correction complete) immediately > > > > + * after setting the 4BITECC_ADD_CALC_START bit. So if you immediately > > > > + * begin trying to poll for the state, you may fall right out of your > > > > + * loop without any of the correction calculations having taken place. > > > > + * The recommendation from the hardware team is to wait till ECC_STATE > > > > + * reads less than 4, which means ECC HW has entered correction state. > > > > + */ > > > > + do { > > > > + ecc_state = (davinci_nand_readl(info, > > > > + NANDFSR_OFFSET) >> 8) & 0x0f; > > > > + cpu_relax(); > > > > + } while ((ecc_state < 4) && time_before(jiffies, timeo)); > > > > > > An up-to-100-milliseond busy wait is pretty bad. For how long do you > > > expect this to spin in practice? > > > > On the hardware, I have never seen this taking 100 msec to come out of > > the loop. I'll check with the hardware folks on the maximum time to wait > > for, before the ECC engine is ready. > > I checked this with the hardware team but no one is sure about the exact > time one should wait before the ECC engine becomes ready. But everyone is > of the opinion that 100 loop cycles should be enough. To be on the safer > side, I'll be changing the timeout to 10 milliseconds in the next version > of this patch. Going from 100ms down to 10ms sounds a bit risky. It'd be better to retain the 100ms and to make the kernel spend most of that time sleeping, rather than busywaiting, IMO.
Hi, On Tue, Jul 13, 2010 at 23:11:26, Andrew Morton wrote: > On Tue, 13 Jul 2010 15:02:59 +0530 "Sudhakar Rajashekhara" <sudhakar.raj@ti.com> wrote: > > > Hi, > > > > On Mon, Jul 12, 2010 at 11:58:18, Sudhakar Rajashekhara wrote: > > > On Sat, Jul 10, 2010 at 04:09:32, Andrew Morton wrote: > > > > On Fri, 9 Jul 2010 10:59:49 +0530 > > > > Sudhakar Rajashekhara <sudhakar.raj@ti.com> wrote: > > > > > > > > > + > > > > > + /* > > > > > + * ECC_STATE field reads 0x3 (Error correction complete) immediately > > > > > + * after setting the 4BITECC_ADD_CALC_START bit. So if you immediately > > > > > + * begin trying to poll for the state, you may fall right out of your > > > > > + * loop without any of the correction calculations having taken place. > > > > > + * The recommendation from the hardware team is to wait till ECC_STATE > > > > > + * reads less than 4, which means ECC HW has entered correction state. > > > > > + */ > > > > > + do { > > > > > + ecc_state = (davinci_nand_readl(info, > > > > > + NANDFSR_OFFSET) >> 8) & 0x0f; > > > > > + cpu_relax(); > > > > > + } while ((ecc_state < 4) && time_before(jiffies, timeo)); > > > > > > > > An up-to-100-milliseond busy wait is pretty bad. For how long do you > > > > expect this to spin in practice? > > > > > > On the hardware, I have never seen this taking 100 msec to come out of > > > the loop. I'll check with the hardware folks on the maximum time to wait > > > for, before the ECC engine is ready. > > > > I checked this with the hardware team but no one is sure about the exact > > time one should wait before the ECC engine becomes ready. But everyone is > > of the opinion that 100 loop cycles should be enough. To be on the safer > > side, I'll be changing the timeout to 10 milliseconds in the next version > > of this patch. > > Going from 100ms down to 10ms sounds a bit risky. It'd be better to > retain the 100ms and to make the kernel spend most of that time > sleeping, rather than busywaiting, IMO. > I chose 100ms timeout randomly, but today I did some calculation using do_gettimeofday() to find out the actual time spent inside the loop. I found that, control is coming out of loop within 5 microseconds. So I'll go ahead and use usecs_to_jiffies(100) as timeout. Busywaiting for such a short duration may not be a problem. Regards, Sudhakar
diff --git a/drivers/mtd/nand/davinci_nand.c b/drivers/mtd/nand/davinci_nand.c index 9c9d893..2ac7367 100644 --- a/drivers/mtd/nand/davinci_nand.c +++ b/drivers/mtd/nand/davinci_nand.c @@ -311,7 +311,9 @@ static int nand_davinci_correct_4bit(struct mtd_info *mtd, unsigned short ecc10[8]; unsigned short *ecc16; u32 syndrome[4]; + u32 ecc_state; unsigned num_errors, corrected; + unsigned long timeo = jiffies + msecs_to_jiffies(100); /* All bytes 0xff? It's an erased page; ignore its ECC. */ for (i = 0; i < 10; i++) { @@ -361,6 +363,21 @@ compare: */ davinci_nand_writel(info, NANDFCR_OFFSET, davinci_nand_readl(info, NANDFCR_OFFSET) | BIT(13)); + + /* + * ECC_STATE field reads 0x3 (Error correction complete) immediately + * after setting the 4BITECC_ADD_CALC_START bit. So if you immediately + * begin trying to poll for the state, you may fall right out of your + * loop without any of the correction calculations having taken place. + * The recommendation from the hardware team is to wait till ECC_STATE + * reads less than 4, which means ECC HW has entered correction state. + */ + do { + ecc_state = (davinci_nand_readl(info, + NANDFSR_OFFSET) >> 8) & 0x0f; + cpu_relax(); + } while ((ecc_state < 4) && time_before(jiffies, timeo)); + for (;;) { u32 fsr = davinci_nand_readl(info, NANDFSR_OFFSET);