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 X-Spam-Level: X-Spam-Status: No, score=-3.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 44C27C48BDF for ; Tue, 22 Jun 2021 14:21:39 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 2C9AE61361 for ; Tue, 22 Jun 2021 14:21:39 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231713AbhFVOXx convert rfc822-to-8bit (ORCPT ); Tue, 22 Jun 2021 10:23:53 -0400 Received: from eu-smtp-delivery-151.mimecast.com ([185.58.85.151]:27783 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231478AbhFVOXu (ORCPT ); Tue, 22 Jun 2021 10:23:50 -0400 Received: from AcuMS.aculab.com (156.67.243.121 [156.67.243.121]) (Using TLS) by relay.mimecast.com with ESMTP id uk-mta-60-ZAfcHqbRPfSLTlwrCStpTA-1; Tue, 22 Jun 2021 15:21:28 +0100 X-MC-Unique: ZAfcHqbRPfSLTlwrCStpTA-1 Received: from AcuMS.Aculab.com (10.202.163.4) by AcuMS.aculab.com (10.202.163.4) with Microsoft SMTP Server (TLS) id 15.0.1497.18; Tue, 22 Jun 2021 15:21:27 +0100 Received: from AcuMS.Aculab.com ([fe80::994c:f5c2:35d6:9b65]) by AcuMS.aculab.com ([fe80::994c:f5c2:35d6:9b65%12]) with mapi id 15.00.1497.018; Tue, 22 Jun 2021 15:21:27 +0100 From: David Laight To: 'Alan Stern' CC: 'Mauro Carvalho Chehab' , "linux-usb@vger.kernel.org" , "linuxarm@huawei.com" , "mauro.chehab@huawei.com" , Laurent Pinchart , Mauro Carvalho Chehab , "linux-kernel@vger.kernel.org" , "linux-media@vger.kernel.org" , "stable@vger.kernel.org" Subject: RE: [PATCH v3] media: uvc: don't do DMA on stack Thread-Topic: [PATCH v3] media: uvc: don't do DMA on stack Thread-Index: AQHXZqL+Rr1YUDB8sUS3jMuLXE5TdKsfq6kggABLhgCAAB2gMA== Date: Tue, 22 Jun 2021 14:21:27 +0000 Message-ID: References: <6832dffafd54a6a95b287c4a1ef30250d6b9237a.1624282817.git.mchehab+huawei@kernel.org> <20210622132922.GB452785@rowland.harvard.edu> In-Reply-To: <20210622132922.GB452785@rowland.harvard.edu> Accept-Language: en-GB, en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-exchange-transport-fromentityheader: Hosted x-originating-ip: [10.202.205.107] MIME-Version: 1.0 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=C51A453 smtp.mailfrom=david.laight@aculab.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: aculab.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Alan Stern > Sent: 22 June 2021 14:29 ... > > Thought... > > > > Is kmalloc(1, GFP_KERNEL) guaranteed to return a pointer into > > a cache line that will not be accessed by any other code? > > (This is slightly weaker than requiring a cache-line aligned > > pointer - but very similar.) > > As I understand it, on architectures that do not have cache-coherent > I/O, kmalloc is guaranteed to return a buffer that is > cacheline-aligned and whose length is a multiple of the cacheline > size. > > Now, whether that buffer ends up being accessed by any other code > depends on what your driver does with the pointer it gets from > kmalloc. :-) Thanks for the clarification. Most of the small allocates in the usb stack are for transmits where it is only necessary to ensure a cache write-back. I know there has been some confusion because one of the allocators can add a small header to every allocation. This can lead to unexpectedly inadequately aligned pointers. If it is updated when the preceding block is freed (as some user-space mallocs do) then it would need to be in a completely separate cache line. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)