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(2603:10a6:208:75::13) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.1143.18; Tue, 25 Sep 2018 11:52:49 +0000 Subject: Re: [BUG] pwm: sysfs: It is not possible to export more than one channel To: Thierry Reding Cc: Fabrice Gasnier , Gottfried Haider , linux-pwm@vger.kernel.org, "linux-kernel@vger.kernel.org" , H Hartley Sweeten References: <20180925102153.GI7097@ulmo> From: =?UTF-8?B?TWljaGFsIFZva8OhxI0=?= Message-ID: <06ae5eac-6474-2eaa-94fe-68c750ef5479@ysoft.com> Date: Tue, 25 Sep 2018 13:52:46 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 MIME-Version: 1.0 In-Reply-To: <20180925102153.GI7097@ulmo> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit X-Originating-IP: [89.24.100.190] X-ClientProxiedBy: AM6PR0202CA0049.eurprd02.prod.outlook.com (2603:10a6:20b:3a::26) To AM0PR04MB4659.eurprd04.prod.outlook.com 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ysoft.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 25 Sep 2018 11:52:49.1967 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 04573ebd-f3ad-44f7-e2ba-08d622dd6acf X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: b5839965-430f-4be2-b282-d7a3149f2b37 X-MS-Exchange-Transport-CrossTenantHeadersStamped: AM0PR04MB4659 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 25.9.2018 12:21, Thierry Reding wrote: > On Tue, Sep 25, 2018 at 11:11:32AM +0200, Michal Vokáč wrote: >> Hi, >> >> Since commit 7e5d1fd75c3d ("pwm: Set class for exported channels in >> sysfs") it is not possible to export more than one PWM channel. >> >> This is because for each exported channel a directory named pwmN is >> created in /sys/class/pwm/. As channels for all PWM chips are numbered >> from 0 it is not possible to export pwmN channel from one chip and >> pwmN channel from another chip at the same time. >> >> In theory if your SoC has N PWM chips and each chip has N channels you >> should be able to export N channels. But only one channel from each chip. >> >> I found the issue on i.MX6dl SoC with two PWM chips where each PWM chip >> has just one channel. >> >> This is how it can be reproduced: >> >> root@hydraco:/sys/class/pwm# ls >> pwmchip0 -> ../../devices/soc0/soc/2000000.aips-bus/2080000.pwm/pwm/pwmchip0 >> pwmchip1 -> ../../devices/soc0/soc/2000000.aips-bus/208c000.pwm/pwm/pwmchip1 >> >> root@hydraco:/sys/class/pwm# cat pwmchip0/npwm >> 1 >> root@hydraco:/sys/class/pwm# cat pwmchip1/npwm >> 1 >> >> root@hydraco:/sys/class/pwm# echo 0 > pwmchip0/export >> root@hydraco:/sys/class/pwm# ls -l >> pwm0 -> ../../devices/soc0/soc/2000000.aips-bus/2080000.pwm/pwm/pwmchip0/pwm0 >> pwmchip0 -> ../../devices/soc0/soc/2000000.aips-bus/2080000.pwm/pwm/pwmchip0 >> pwmchip1 -> ../../devices/soc0/soc/2000000.aips-bus/208c000.pwm/pwm/pwmchip1 >> >> root@hydraco:/sys/class/pwm# echo 0 > pwmchip1/export >> [ 543.110824] sysfs: cannot create duplicate filename '/class/pwm/pwm0' >> [ 543.117314] CPU: 1 PID: 351 Comm: sh Not tainted 4.19.0-rc5-00025-g249f3ed-dirty #8 >> [ 543.124990] Hardware name: Freescale i.MX6 Quad/DualLite (Device Tree) >> [ 543.131587] [<80112d20>] (unwind_backtrace) from [<8010ddb4>] (show_stack+0x20/0x24) >> [ 543.139383] [<8010ddb4>] (show_stack) from [<80ba1044>] (dump_stack+0x80/0x94) >> [ 543.146652] [<80ba1044>] (dump_stack) from [<8031e2b4>] (sysfs_warn_dup+0x6c/0x78) >> [ 543.154257] [<8031e2b4>] (sysfs_warn_dup) from [<8031e61c>] (sysfs_do_create_link_sd+0xd8/0xdc) >> [ 543.162988] [<8031e61c>] (sysfs_do_create_link_sd) from [<8031e678>] (sysfs_create_link+0x38/0x44) >> [ 543.171986] [<8031e678>] (sysfs_create_link) from [<806026b0>] (device_add+0x2c8/0x630) >> [ 543.180025] [<806026b0>] (device_add) from [<80602a3c>] (device_register+0x24/0x28) >> [ 543.187724] [<80602a3c>] (device_register) from [<80501acc>] (export_store+0x118/0x190) >> [ 543.195762] [<80501acc>] (export_store) from [<805ffc10>] (dev_attr_store+0x28/0x34) >> [ 543.203539] [<805ffc10>] (dev_attr_store) from [<8031d7dc>] (sysfs_kf_write+0x48/0x54) >> [ 543.211485] [<8031d7dc>] (sysfs_kf_write) from [<8031cde0>] (kernfs_fop_write+0xf8/0x1e0) >> [ 543.219696] [<8031cde0>] (kernfs_fop_write) from [<80294d78>] (__vfs_write+0x48/0x170) >> [ 543.227645] [<80294d78>] (__vfs_write) from [<80295064>] (vfs_write+0xb4/0x1c0) >> [ 543.234981] [<80295064>] (vfs_write) from [<802952e8>] (ksys_write+0x5c/0xbc) >> [ 543.242144] [<802952e8>] (ksys_write) from [<80295360>] (sys_write+0x18/0x1c) >> [ 543.249313] [<80295360>] (sys_write) from [<80101000>] (ret_fast_syscall+0x0/0x54) >> [ 543.256901] Exception stack(0xecfbffa8 to 0xecfbfff0) >> [ 543.261980] ffa0: 00000002 76fc8000 00000001 76fc8000 00000002 00000000 >> [ 543.270184] ffc0: 00000002 76fc8000 76f5cd60 00000004 00000002 000bd134 00000001 000ba730 >> [ 543.278380] ffe0: 00000000 7ebff954 76e8b988 76ee3cd0 >> -sh: echo: write error: File exists >> >> I am not sure what will be the right solution. The problem is that >> numbering of the PWM channels does not work the same way as in the GPIO >> subsystem where each GPIO has its unique number. >> >> Should we just revert the change? It was not documented in the PWM sysfs >> interface. What do you think? > > Fabrice has reported the same thing. We've been discussing possible > solutions here: > > http://patchwork.ozlabs.org/patch/973224/ Thank you for the reference Thierry. I was convinced there must be someone else who encountered the same problem. My search did not produce anything useful though. > It looks to me like we'd need to at least revert the patch to restore > the sysfs ABI. At the same time I think we'll want to fix the issue that > the offending commit was trying to address. Agree, ideally we want to solve both problems. My knowledge of the overall architecture is still fairly limited though. I have no idea if/how we can deal with the issue the offending commit was addressing if we revert it. Would be great if we could somehow choose a different name just for the symlink but leave the device name as is to not break the ABI as Fabrice suggested. Michal