[{"id":3686139,"web_url":"http://patchwork.ozlabs.org/comment/3686139/","msgid":"","date":"2026-05-05T05:29:29","subject":"Re: [PATCH net] net: wan: fsl_uhdlc_hdlc: fix dma_rmb usage in\n hdlc_rx_done","submitter":{"id":92089,"url":"http://patchwork.ozlabs.org/api/people/92089/","name":"Christophe Leroy (CS GROUP)","email":"chleroy@kernel.org"},"content":"Hi,\n\nLe 04/05/2026 à 17:56, Holger Brunck a écrit :\n> If dma_rmb is used it has to be done after reading bd_status and checking\n> if R_E_S is zero. Therefore we need to move it into the while loop.\n\nCan you give more details ? Why does dma_rmb() has to be done after \nreading bd_status and checking if R_E_S is zero ?\n\n> \n> Fixes: c19b6d246a35 (\"drivers/net: support hdlc function for QE-UCC\")\n> Signed-off-by: Holger Brunck \n> ---\n> drivers/net/wan/fsl_ucc_hdlc.c | 3 +--\n> 1 file changed, 1 insertion(+), 2 deletions(-)\n> \n> diff --git a/drivers/net/wan/fsl_ucc_hdlc.c b/drivers/net/wan/fsl_ucc_hdlc.c\n> index 15bfb78381d4..09081f128a98 100644\n> --- a/drivers/net/wan/fsl_ucc_hdlc.c\n> +++ b/drivers/net/wan/fsl_ucc_hdlc.c\n> @@ -523,12 +523,12 @@ static int hdlc_rx_done(struct ucc_hdlc_private *priv, int rx_work_limit)\n> \tu16 length, howmany = 0;\n> \tu8 *bdbuffer;\n> \n> -\tdma_rmb();\n> \tbd = priv->currx_bd;\n> \tbd_status = be16_to_cpu(bd->status);\n> \n> \t/* while there are received buffers and BD is full (~R_E) */\n> \twhile (!((bd_status & (R_E_S)) || (--rx_work_limit < 0))) {\n> +\t\tdma_rmb();\n> \t\tif (bd_status & (RX_BD_ERRORS)) {\n> \t\t\tdev->stats.rx_errors++;\n> \n> @@ -610,7 +610,6 @@ static int hdlc_rx_done(struct ucc_hdlc_private *priv, int rx_work_limit)\n> \n> \t\tbd_status = be16_to_cpu(bd->status);\n> \t}\n> -\tdma_rmb();\n> \n> \tpriv->currx_bd = bd;\n> \treturn howmany;","headers":{"Return-Path":"\n ","X-Original-To":["incoming@patchwork.ozlabs.org","linuxppc-dev@lists.ozlabs.org"],"Delivered-To":"patchwork-incoming@legolas.ozlabs.org","Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256\n header.s=k20201202 header.b=hpzC+qsQ;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=lists.ozlabs.org\n (client-ip=2404:9400:21b9:f100::1; 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charset=UTF-8; format=flowed","Content-Transfer-Encoding":"8bit","X-Spam-Status":"No, score=-0.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED,\n\tDKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,SPF_HELO_NONE,SPF_PASS\n\tautolearn=disabled version=4.0.1 OzLabs 8","X-Spam-Checker-Version":"SpamAssassin 4.0.1 (2024-03-25) on lists.ozlabs.org"}},{"id":3686202,"web_url":"http://patchwork.ozlabs.org/comment/3686202/","msgid":"","date":"2026-05-05T08:14:31","subject":"RE: [PATCH net] net: wan: fsl_uhdlc_hdlc: fix dma_rmb usage in\n hdlc_rx_done","submitter":{"id":82682,"url":"http://patchwork.ozlabs.org/api/people/82682/","name":"Holger Brunck","email":"holger.brunck@hitachienergy.com"},"content":"> \n> Le 04/05/2026 à 17:56, Holger Brunck a écrit :\n> > If dma_rmb is used it has to be done after reading bd_status and\n> > checking if R_E_S is zero. Therefore we need to move it into the while loop.\n> \n> Can you give more details ? Why does dma_rmb() has to be done after reading\n> bd_status and checking if R_E_S is zero ?\n> \n\nwhen R_E_S is zero in the status of the buffer descriptor it means the buffer is\nfilled with data from the device. Now the CPU owns the descriptor. Now we\nshould execute the dma_rmb to be sure that we read the data correctly. \nAnd this we need to redo for each buffer descriptor which is filled with data,\nthat’s why it must be done within the for loop and not before and after.\n\nThis is also consistent with the example in Documentation/memory-barriers.txt\n\nBest regards\nHolger","headers":{"Return-Path":"\n ","X-Original-To":["incoming@patchwork.ozlabs.org","linuxppc-dev@lists.ozlabs.org"],"Delivered-To":"patchwork-incoming@legolas.ozlabs.org","Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=hitachienergy.com header.i=@hitachienergy.com\n header.a=rsa-sha256 header.s=selector1 header.b=aXIiDLvx;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=lists.ozlabs.org\n (client-ip=112.213.38.117; helo=lists.ozlabs.org;\n envelope-from=linuxppc-dev+bounces-20441-incoming=patchwork.ozlabs.org@lists.ozlabs.org;\n receiver=patchwork.ozlabs.org)","lists.ozlabs.org;\n arc=pass smtp.remote-ip=\"2a01:111:f403:c207::3\" arc.chain=microsoft.com","lists.ozlabs.org;\n dmarc=pass (p=quarantine dis=none) header.from=hitachienergy.com","lists.ozlabs.org;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=hitachienergy.com header.i=@hitachienergy.com\n header.a=rsa-sha256 header.s=selector1 header.b=aXIiDLvx;\n\tdkim-atps=neutral","lists.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=hitachienergy.com\n (client-ip=2a01:111:f403:c207::3;\n helo=mrwpr03cu001.outbound.protection.outlook.com;\n envelope-from=holger.brunck@hitachienergy.com; 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charset=\"utf-8\"","Content-Transfer-Encoding":"base64","X-Mailing-List":"linuxppc-dev@lists.ozlabs.org","List-Id":"","List-Help":"","List-Owner":"","List-Post":"","List-Archive":",\n ","List-Subscribe":",\n ,\n ","List-Unsubscribe":"","Precedence":"list","MIME-Version":"1.0","X-OriginatorOrg":"hitachienergy.com","X-MS-Exchange-CrossTenant-AuthAs":"Internal","X-MS-Exchange-CrossTenant-AuthSource":"AM0PR06MB10396.eurprd06.prod.outlook.com","X-MS-Exchange-CrossTenant-Network-Message-Id":"\n 8a0af3ed-483a-4dcf-f0bf-08deaa7e55fe","X-MS-Exchange-CrossTenant-originalarrivaltime":"05 May 2026 08:14:31.8091\n (UTC)","X-MS-Exchange-CrossTenant-fromentityheader":"Hosted","X-MS-Exchange-CrossTenant-id":"7831e6d9-dc6c-4cd1-9ec6-1dc2b4133195","X-MS-Exchange-CrossTenant-mailboxtype":"HOSTED","X-MS-Exchange-CrossTenant-userprincipalname":"\n OutrD3hbJ4+TfQETj8hzZ5e+Sl0q0r90MHq1X2m7NAYhSdhraxP8V33bcJoALlVEK6R0Gs+i22lAM/faqmJ5EWWzzLRRRCgyxSq1iA01WAc=","X-MS-Exchange-Transport-CrossTenantHeadersStamped":"AM8PR06MB7361","X-Spam-Status":"No, score=-0.2 required=3.0 tests=DKIM_SIGNED,DKIM_VALID,\n\tDKIM_VALID_AU,DKIM_VALID_EF,SPF_HELO_PASS,SPF_PASS autolearn=disabled\n\tversion=4.0.1 OzLabs 8","X-Spam-Checker-Version":"SpamAssassin 4.0.1 (2024-03-25) on lists.ozlabs.org"}},{"id":3686228,"web_url":"http://patchwork.ozlabs.org/comment/3686228/","msgid":"<07fc37e5-7815-48ed-8632-62e896f8f6ad@kernel.org>","date":"2026-05-05T08:37:30","subject":"Re: [PATCH net] net: wan: fsl_uhdlc_hdlc: fix dma_rmb usage in\n hdlc_rx_done","submitter":{"id":92089,"url":"http://patchwork.ozlabs.org/api/people/92089/","name":"Christophe Leroy (CS GROUP)","email":"chleroy@kernel.org"},"content":"Le 05/05/2026 à 10:14, Holger Brunck a écrit :\n>>\n>> Le 04/05/2026 à 17:56, Holger Brunck a écrit :\n>>> If dma_rmb is used it has to be done after reading bd_status and\n>>> checking if R_E_S is zero. Therefore we need to move it into the while loop.\n>>\n>> Can you give more details ? Why does dma_rmb() has to be done after reading\n>> bd_status and checking if R_E_S is zero ?\n>>\n> \n> when R_E_S is zero in the status of the buffer descriptor it means the buffer is\n> filled with data from the device. Now the CPU owns the descriptor. Now we\n> should execute the dma_rmb to be sure that we read the data correctly.\n> And this we need to redo for each buffer descriptor which is filled with data,\n> that’s why it must be done within the for loop and not before and after.\n\nWe enter hdlc_rx_done() after an interrupt which triggers scheduling of \nucc_hdlc_poll(). I think dma_rmb() is needed _before_ reading the first \nstatus, otherwise it might read an erroneous status.\n\nOnce we are here the interrupt has been cleared so any new buffer will \ntrigger a new interrupt and call again this function. Therefore I don't \nthink it is worth the cost of a dma_rmb() inside the loop.\n\nChristophe","headers":{"Return-Path":"\n ","X-Original-To":["incoming@patchwork.ozlabs.org","linuxppc-dev@lists.ozlabs.org"],"Delivered-To":"patchwork-incoming@legolas.ozlabs.org","Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256\n header.s=k20201202 header.b=iCV7vR2c;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=lists.ozlabs.org\n (client-ip=2404:9400:21b9:f100::1; helo=lists.ozlabs.org;\n envelope-from=linuxppc-dev+bounces-20445-incoming=patchwork.ozlabs.org@lists.ozlabs.org;\n receiver=patchwork.ozlabs.org)","lists.ozlabs.org;\n arc=none smtp.remote-ip=172.105.4.254","lists.ozlabs.org;\n dmarc=pass (p=quarantine dis=none) header.from=kernel.org","lists.ozlabs.org;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256\n header.s=k20201202 header.b=iCV7vR2c;\n\tdkim-atps=neutral","lists.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=kernel.org\n (client-ip=172.105.4.254; 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charset=UTF-8; format=flowed","Content-Transfer-Encoding":"8bit","X-Spam-Status":"No, score=-0.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED,\n\tDKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,SPF_HELO_NONE,SPF_PASS\n\tautolearn=disabled version=4.0.1 OzLabs 8","X-Spam-Checker-Version":"SpamAssassin 4.0.1 (2024-03-25) on lists.ozlabs.org"}},{"id":3686306,"web_url":"http://patchwork.ozlabs.org/comment/3686306/","msgid":"","date":"2026-05-05T10:11:45","subject":"RE: [PATCH net] net: wan: fsl_uhdlc_hdlc: fix dma_rmb usage in\n hdlc_rx_done","submitter":{"id":82682,"url":"http://patchwork.ozlabs.org/api/people/82682/","name":"Holger Brunck","email":"holger.brunck@hitachienergy.com"},"content":"> \n> Le 05/05/2026 à 10:14, Holger Brunck a écrit :\n> >>\n> >> Le 04/05/2026 à 17:56, Holger Brunck a écrit :\n> >>> If dma_rmb is used it has to be done after reading bd_status and\n> >>> checking if R_E_S is zero. Therefore we need to move it into the while loop.\n> >>\n> >> Can you give more details ? Why does dma_rmb() has to be done after\n> >> reading bd_status and checking if R_E_S is zero ?\n> >>\n> >\n> > when R_E_S is zero in the status of the buffer descriptor it means the\n> > buffer is filled with data from the device. Now the CPU owns the\n> > descriptor. Now we should execute the dma_rmb to be sure that we read the\n> data correctly.\n> > And this we need to redo for each buffer descriptor which is filled\n> > with data, that’s why it must be done within the for loop and not before and\n> after.\n> \n> We enter hdlc_rx_done() after an interrupt which triggers scheduling of\n> ucc_hdlc_poll(). I think dma_rmb() is needed _before_ reading the first status,\n> otherwise it might read an erroneous status.\n> \n\nwe end up in hdlc_rx_done also if a TX interrupt was triggered due to the NAPI layer.\nIf RX bd_status is still 1 we would read a pending RX packet next time. So the dma_rmb()\nbefore is not necessary. But if we have read a zero in bd_status we need to be sure that\nthe data written from the QE to RAM is fully available. That is why I think doing it after\nreading the status is correct here.\n \n> Once we are here the interrupt has been cleared so any new buffer will trigger a\n> new interrupt and call again this function. Therefore I don't think it is worth the\n> cost of a dma_rmb() inside the loop.\n> \n\nYes, but while we are in the for loop the QE might have written the next packet to\nthe memory in background and sets the bd_status to zero. And if this is the\ncase we need again make sure that the memory we read is correct.\n\nFor example the following driver does the same AFAIU.\ndrivers/net/ethernet/intel/e1000e/netdev.c \nfrom line 1524 ff\n\nIt first reads the status for the RX packet then does dma_rmb() and then reads\nthe data itself. And it is done within the while loop as any further available packet\nmay have arrived just now and we need to be sure that the data is valid. \n\nBest regards\nHolger","headers":{"Return-Path":"\n ","X-Original-To":["incoming@patchwork.ozlabs.org","linuxppc-dev@lists.ozlabs.org"],"Delivered-To":"patchwork-incoming@legolas.ozlabs.org","Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=hitachienergy.com header.i=@hitachienergy.com\n header.a=rsa-sha256 header.s=selector1 header.b=rceZgSXk;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=lists.ozlabs.org\n (client-ip=2404:9400:21b9:f100::1; helo=lists.ozlabs.org;\n envelope-from=linuxppc-dev+bounces-20475-incoming=patchwork.ozlabs.org@lists.ozlabs.org;\n receiver=patchwork.ozlabs.org)","lists.ozlabs.org;\n arc=pass smtp.remote-ip=\"2a01:111:f403:c200::5\" arc.chain=microsoft.com","lists.ozlabs.org;\n dmarc=pass (p=quarantine dis=none) header.from=hitachienergy.com","lists.ozlabs.org;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=hitachienergy.com header.i=@hitachienergy.com\n header.a=rsa-sha256 header.s=selector1 header.b=rceZgSXk;\n\tdkim-atps=neutral","lists.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=hitachienergy.com\n (client-ip=2a01:111:f403:c200::5;\n helo=duzpr83cu001.outbound.protection.outlook.com;\n envelope-from=holger.brunck@hitachienergy.com; 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