From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 08CA7C433EF for ; Wed, 22 Jun 2022 16:01:21 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1359731AbiFVQBQ (ORCPT ); Wed, 22 Jun 2022 12:01:16 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:35270 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1376321AbiFVQAx (ORCPT ); Wed, 22 Jun 2022 12:00:53 -0400 Received: from mail-yw1-f171.google.com (mail-yw1-f171.google.com [209.85.128.171]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id BDDFE30F71; Wed, 22 Jun 2022 08:59:53 -0700 (PDT) Received: by mail-yw1-f171.google.com with SMTP id 00721157ae682-3177e60d980so145798167b3.12; Wed, 22 Jun 2022 08:59:53 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=kpNwZhljCOrWIGTt6F/gaTTGQN5fTFFddc0JAHcw+w8=; b=Y63UvV5XOSAwU1DrslFQ73plK0TrS3H8cfOskX9FuTkTLQnHBfoJCO96n0w0xTL1hJ x53WElW72qPPT4zZNkWNsZtQGhmKYrpj6DQ+pUvcB7jKQeZRkZa59OuCl34RoVuhdQK3 ocpmq3VWnM8a8cxVo2DGVpcrleCtu8g3dvKwHR/YMlEJRXk/cVnZ0u8CnCX6yUc4QEAM WtCkjbcCIQFkxJxt+lonbYo2ZKI5GHaMQ4Tv065sY/g5XXsv4GWXiA4mMEsGqhTtfDQz KECIl3J8Dophi0wXRahK8eH0u3fxvppMbITn3jJocb1ZKfJ5PGv4IcfGvtFqBbpWkXOX XuRA== X-Gm-Message-State: AJIora9+I6xTtxazNAbNsYlt3TJLYcEYZ+550c39ygmNqsI1NBv97A7h prADNDiHri5Um8qiZnLNkxNJwLmyC4Y4vJdAbo8= X-Google-Smtp-Source: AGRyM1spkCdgUtepS+t4kJYH8jGNlhGdJN7N86AhkObEYU7+CIDfp7npMA8MccDcsvZMR9jgtdZ5cJepF6/rtoRq258= X-Received: by 2002:a81:8283:0:b0:314:48f4:17d0 with SMTP id s125-20020a818283000000b0031448f417d0mr5090043ywf.220.1655913592832; Wed, 22 Jun 2022 08:59:52 -0700 (PDT) MIME-Version: 1.0 References: <20220610213335.3077375-2-rhett.aultman@samsara.com> <20220611153104.sksoxn4dmo5rgnk3@pengutronix.de> <48caa879b0064ced97623bf1dad5b2d9@AcuMS.aculab.com> In-Reply-To: From: Vincent MAILHOL Date: Thu, 23 Jun 2022 00:59:41 +0900 Message-ID: Subject: Re: [PATCH v3 1/2] drivers: usb/core/urb: Add URB_FREE_COHERENT To: Greg Kroah-Hartman Cc: David Laight , Alan Stern , Marc Kleine-Budde , Rhett Aultman , "linux-usb@vger.kernel.org" , linux-can , Oliver Neukum Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-can@vger.kernel.org On Wed. 22 Jun 2022 at 21:24, Greg Kroah-Hartman wrote: > On Wed, Jun 22, 2022 at 07:34:57PM +0900, Vincent MAILHOL wrote: > > On Wed. 22 Jun 2022 at 18:44, Greg Kroah-Hartman > > wrote: > > > On Wed, Jun 22, 2022 at 09:22:12AM +0000, David Laight wrote: > > > > From: Vincent MAILHOL > > > > > Sent: 21 June 2022 16:56 > > > > > > > > > > On Wed. 22 Jun 2022 at 00:13, Alan Stern wrote: > > > > > > On Tue, Jun 21, 2022 at 11:59:16PM +0900, Vincent MAILHOL wrote: > > > > > > > I (probably wrongly) assumed that urb::transfer_buffer_length was the > > > > > > > allocated length and urb::actual_length was what was actually being > > > > > > > transferred. Right now, I am just confused. Seems that I need to study > > > > > > > a bit more and understand the real purpose of > > > > > > > urb::transfer_buffer_length because I still fail to understand in > > > > > > > which situation this can be different from the allocated length. > > > > > > > > > > > > urb->transfer_buffer_length is the amount of data that the driver wants > > > > > > to send or expects to receive. urb->actual_length is the amount of data > > > > > > that was actually sent or actually received. > > > > > > > > > > > > Neither of these values has to be the same as the size of the buffer -- > > > > > > but they better not be bigger! > > > > > > > > > > Thanks. Now things are a bit clearer. > > > > > I guess that for the outcoming URB what I proposed made no sense. For > > > > > incoming URB, I guess that most of the drivers want to set > > > > > urb::transfer_buffer once for all with the allocated size and never > > > > > touch it again. > > > > > > > > > > Maybe the patch only makes sense of the incoming URB. Would it make > > > > > sense to keep it but with an additional check to trigger a dmesg > > > > > warning if this is used on an outcoming endpoint and with additional > > > > > comment that the URB_FREE_COHERENT requires urb::transfer_buffer to be > > > > > the allocated size? > > > > > > > > IIRC urb are pretty big. > > > > > > What exactly do you mean by "pretty big" here? And what is wrong with > > > that, I have never seen any issues with the current size of that > > > structure in any benchmark or performance results. All USB bottlenecks > > > that I know of are either in the hardware layer, or in the protocol > > > layer itself (i.e. usb-storage protocol). > > > > > > > You'd be unlucky if adding an extra field to hold the allocated > > > > size would ever need more memory. > > > > So it might just be worth saving the allocated size. > > > > > > Maybe, yes, then we could transition to allocating the urb and buffer at > > > the same time like we do partially for iso streams in an urb. But that > > > still might be overkill for just this one driver. > > > > Well, I wouldn't have proposed the patch if it only applied to a > > single driver. If we add a urb::allocated_transfer_size as suggested > > by David, I believe that the majority of the drivers using DMA memory > > will be able to rely on that URB_FREE_COHERENT flag for the garbage > > collection. > > > > The caveat, as you pointed before, is that the developper still needs > > to be aware of the limitations of DMA and that it should not be freed > > in an IRQ context. e.g. no call to usb_kill_anchored_urbs() or other > > functions that would lead to urb_destroy(). > > > > > I'm curious as to why > > > a slow and tiny protocol like CAN needs to use usb_alloc_coherent() for > > > its buffers in the first place. > > > > The CAN protocol, in its latest revision, allows for transfer speed up > > to ~5Mbits. For low performance CPUs, this starts to be a significant > > load. Also, the CAN PDU being small (0 to 64 bytes), many small > > transfers occur. > > And is the memcpy the actual issue here? Even tiny cpus can do large > and small memcopy very very very fast. > > > Unfortunately I did not do any benchmark myself so I won't be able to > > back my explanation with numbers. > > That might be the simplest solution here :) Yes, this would give a clear answer whether or not DMA was needed in the first place. But I do not own that gs_usb device to do the benchmark myself (and to be honest I do not have time to dedicate for this at the moment, maybe I will do it later on some other devices). Has anyone from the linux-can mailing list ever done such a benchmark? Else, is there anyone who would like to volunteer? Yours sincerely, Vincent Mailhol