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 29F6BC433FE for ; Thu, 27 Jan 2022 11:44:27 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S240740AbiA0Lo0 (ORCPT ); Thu, 27 Jan 2022 06:44:26 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:38890 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232131AbiA0LoZ (ORCPT ); Thu, 27 Jan 2022 06:44:25 -0500 Received: from mail-wr1-x433.google.com (mail-wr1-x433.google.com [IPv6:2a00:1450:4864:20::433]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id EA248C061714; Thu, 27 Jan 2022 03:44:24 -0800 (PST) Received: by mail-wr1-x433.google.com with SMTP id w11so4266574wra.4; Thu, 27 Jan 2022 03:44:24 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; h=message-id:date:mime-version:user-agent:subject:content-language:to :references:from:in-reply-to:content-transfer-encoding; bh=LdvVnzVP+ambH9LK42MFqP+4gobIMIEV2AtnE2m4+4w=; b=AO2Ubp21T4K3UXrE4ox8RrOuzjwwgrlPkeKckx6Grh9RDZ/sTc4sbmYVdr0jjAzpTw QVQQPpq97aK2jW5XFd7bQnzTVqoBJJj6378shYn3d2cksVOP7/4zyAiwlP+kuExwEhDw XZJrGcwnFl9NeV9+LhDnIPEtlKSJuAh2tjefqYKPD21aILv8pPHehPvOIQyGlgjdFsPj oMzJiea1Io27zOiupzkuaNpJcLwNMgIk7MJ6504aaHlBk3VjA7u9dBrKb0HD/gWe/+pQ RvbtkX3j16vPwnewh6iIwT26ia4TZVLMcK3m4cq6+CdyMByCBzP2UMRi0DkN5ke2sf1n 11WQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:message-id:date:mime-version:user-agent:subject :content-language:to:references:from:in-reply-to :content-transfer-encoding; bh=LdvVnzVP+ambH9LK42MFqP+4gobIMIEV2AtnE2m4+4w=; b=k6gc6e3VY79WHjK+Qv87cR9HK8/28tg9F8Y56ph87pk//OmlTdH+kCk6xX4pcs5nNm 5fUJ2Mhpe9TC/Q/iB4/Y2s5wZBZqG7Ky/JzTrCV9PgDDJNy0M0oE2po8c4Qglh+ACgGB wjMNZPHCeuJPtYsOneuC06EtIkWUi3jsO+V3W/mrZbPsdDSxvGDujCxZlqlm7gUkcd4E GZXMRWA2685u6fAWc/x5SpAisR+h9lBCRsH06l0zAdJHugCOMRjwMl2+Vja4/DIBlrpj H7TQTSd25gXH8VXXL3T1Gw9ounFWEgWNuj8veg5h2Byau3WuWFFvLDetrIoVEoZzjwxT JfBA== X-Gm-Message-State: AOAM533NlqfET6XZ5IeUy7TFl13VWmtqKDeRFmQWMq+raHE1C2H1C8jy 3t+pjrGaRRnLka7i4ttCIAM= X-Google-Smtp-Source: ABdhPJx/KQtwKWNuH/SNqZ6CP2qgfsi4lZk1rb4KUhwVnUiM+JCFI4ynUa/ZWR4g7iYh+f08Ls5ePQ== X-Received: by 2002:adf:a410:: with SMTP id d16mr2795659wra.517.1643283863392; Thu, 27 Jan 2022 03:44:23 -0800 (PST) Received: from [192.168.178.21] (p57b0bff8.dip0.t-ipconnect.de. [87.176.191.248]) by smtp.gmail.com with ESMTPSA id a6sm1918382wrx.101.2022.01.27.03.44.22 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 27 Jan 2022 03:44:22 -0800 (PST) Message-ID: <50cf1f2f-3fb2-8abb-7497-dafcd97935f3@gmail.com> Date: Thu, 27 Jan 2022 12:44:21 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.5.0 Subject: Re: [Linaro-mm-sig] Re: [Intel-gfx] [PATCH 02/19] dma-buf-map: Add helper to initialize second map Content-Language: en-US To: =?UTF-8?Q?Christian_K=c3=b6nig?= , Lucas De Marchi , linaro-mm-sig@lists.linaro.org, intel-gfx@lists.freedesktop.org, linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org, linux-media@vger.kernel.org References: <20220126203702.1784589-1-lucas.demarchi@intel.com> <20220126203702.1784589-3-lucas.demarchi@intel.com> <20220127075728.ygwgorhnrwaocdqv@ldmartin-desk2> <3066c6a7-fc73-d34d-d209-a3ff6818dfb6@amd.com> <20220127093332.wnkd2qy4tvwg5i5l@ldmartin-desk2> <27aed6b1-b465-6a52-2b0a-d748c9798414@amd.com> From: =?UTF-8?Q?Christian_K=c3=b6nig?= In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Am 27.01.22 um 12:16 schrieb Daniel Vetter: > On Thu, Jan 27, 2022 at 11:21:20AM +0100, Christian König wrote: >> Am 27.01.22 um 11:00 schrieb Daniel Vetter: >>> On Thu, Jan 27, 2022 at 01:33:32AM -0800, Lucas De Marchi wrote: >>>> On Thu, Jan 27, 2022 at 09:57:25AM +0100, Daniel Vetter wrote: >>>>> On Thu, Jan 27, 2022 at 09:02:54AM +0100, Christian König wrote: >>>>>> Am 27.01.22 um 08:57 schrieb Lucas De Marchi: >>>>>>> On Thu, Jan 27, 2022 at 08:27:11AM +0100, Christian König wrote: >>>>>>>> Am 26.01.22 um 21:36 schrieb Lucas De Marchi: >>>>>>>>> When dma_buf_map struct is passed around, it's useful to be able to >>>>>>>>> initialize a second map that takes care of reading/writing to an offset >>>>>>>>> of the original map. >>>>>>>>> >>>>>>>>> Add a helper that copies the struct and add the offset to the proper >>>>>>>>> address. >>>>>>>> Well what you propose here can lead to all kind of problems and is >>>>>>>> rather bad design as far as I can see. >>>>>>>> >>>>>>>> The struct dma_buf_map is only to be filled in by the exporter and >>>>>>>> should not be modified in this way by the importer. >>>>>>> humn... not sure if I was  clear. There is no importer and exporter here. >>>>>> Yeah, and exactly that's what I'm pointing out as problem here. >>>>>> >>>>>> You are using the inter driver framework for something internal to the >>>>>> driver. That is an absolutely clear NAK! >>>>>> >>>>>> We could discuss that, but you guys are just sending around patches to do >>>>>> this without any consensus that this is a good idea. >>>>> Uh I suggested this, also we're already using dma_buf_map all over the >>>>> place as a convenient abstraction. So imo that's all fine, it should allow >>>>> drivers to simplify some code where on igpu it's in normal kernel memory >>>>> and on dgpu it's behind some pci bar. >>>>> >>>>> Maybe we should have a better name for that struct (and maybe also a >>>>> better place), but way back when we discussed that bikeshed I didn't come >>>>> up with anything better really. >>>> I suggest iosys_map since it abstracts access to IO and system memory. >>>> >>>>>>> There is a role delegation on filling out and reading a buffer when >>>>>>> that buffer represents a struct layout. >>>>>>> >>>>>>> struct bla { >>>>>>>     int a; >>>>>>>     int b; >>>>>>>     int c; >>>>>>>     struct foo foo; >>>>>>>     struct bar bar; >>>>>>>     int d; >>>>>>> } >>>>>>> >>>>>>> >>>>>>> This implementation allows you to have: >>>>>>> >>>>>>>     fill_foo(struct dma_buf_map *bla_map) { ... } >>>>>>>     fill_bar(struct dma_buf_map *bla_map) { ... } >>>>>>> >>>>>>> and the first thing these do is to make sure the map it's pointing to >>>>>>> is relative to the struct it's supposed to write/read. Otherwise you're >>>>>>> suggesting everything to be relative to struct bla, or to do the same >>>>>>> I'm doing it, but IMO more prone to error: >>>>>>> >>>>>>>     struct dma_buf_map map = *bla_map; >>>>>>>     dma_buf_map_incr(map, offsetof(...)); >>>>> Wrt the issue at hand I think the above is perfectly fine code. The idea >>>>> with dma_buf_map is really that it's just a special pointer, so writing >>>>> the code exactly as pointer code feels best. Unfortunately you cannot make >>>>> them typesafe (because of C), so the code sometimes looks a bit ugly. >>>>> Otherwise we could do stuff like container_of and all that with >>>>> typechecking in the macros. >>>> I had exactly this code above, but after writting quite a few patches >>>> using it, particularly with functions that have to write to 2 maps (see >>>> patch 6 for example), it felt much better to have something to >>>> initialize correctly from the start >>>> >>>> struct dma_buf_map other_map = *bla_map; >>>> /* poor Lucas forgetting dma_buf_map_incr(map, offsetof(...)); */ >>>> >>>> is error prone and hard to debug since you will be reading/writting >>>> from/to another location rather than exploding >>>> >>>> While with the construct below >>>> >>>> other_map; >>>> ... >>>> other_map = INITIALIZER() >>>> >>>> I can rely on the compiler complaining about uninitialized var. And >>>> in most of the cases I can just have this single line in the beggining of the >>>> function when the offset is constant: >>>> >>>> struct dma_buf_map other_map = INITIALIZER(bla_map, offsetof(..)); >>> Hm yeah that's a good point that this allows us to rely on the compiler to >>> check for uninitialized variables. >>> >>> Maybe include the above (with editing, but keeping the examples) in the >>> kerneldoc to explain why/how to use this? With that the concept at least >>> has my >>> >>> Acked-by: Daniel Vetter >>> >>> I'll leave it up to you & Christian to find a prettier color choice for >>> the naming bikeshed. >> There is one major issue remaining with this and that is dma_buf_vunmap(): >> >> void dma_buf_vunmap(struct dma_buf *dmabuf, struct dma_buf_map *map); >> >> Here we expect the original pointer as returned by dma_buf_map(), otherwise >> we vunmap() the wrong area! >> >> For all TTM based driver this doesn't matter since we keep the vmap base >> separately in the BO anyway (IIRC), but we had at least one case where this >> made boom last year. > Yeah but isn't that the same if it's just a void *? > > If you pass the wrong pointer to an unmap function and not exactly what > you go from the map function, then things go boom. This is like > complaining that the following code wont work > > u32 *stuff > > stuff = kmap_local(some_page); > *stuff++ = 0; > *stuff = 1; > kunmap_locak(stuff); > > It's just ... don't do that :-) Also since we pass dma_buf_map by value > and not by pointer anywhere, the risk of this happening is pretty low > since you tend to work on a copy. Same with void * pointers really. > > Now if people start to pass around struct dma_buf_map * as pointers for > anything else than out parameters, then we're screwed. But that's like > passing around void ** for lolz, which is just wrong (except when it's an > out parameter or actually an array of pointers ofc). > > Or I really don't get your concern and you mean something else? No that's pretty much it. It's just that we hide the pointer inside a structure and it is absolutely not obvious to a driver dev that you can't do: dma_buf_vmap(.., &map); dma_buf_map_inr(&map, x); dma_buf_vunmap(.., &map); As bare minimum I strongly suggest that we add some WARN_ONs to the framework to check that the pointer given to dma_buf_vunmap() is at least page aligned. Christian. > -Daniel > > >> Christian. >> >>> -Daniel >>> >>>> Lucas De Marchi >>>> >>>>> -Daniel >>>>> >>>>>>> IMO this construct is worse because at a point in time in the function >>>>>>> the map was pointing to the wrong thing the function was supposed to >>>>>>> read/write. >>>>>>> >>>>>>> It's also useful when the function has double duty, updating a global >>>>>>> part of the struct and a table inside it (see example in patch 6) >>>>>>> >>>>>>> thanks >>>>>>> Lucas De Marchi >>>>> -- >>>>> Daniel Vetter >>>>> Software Engineer, Intel Corporation >>>>> https://nam11.safelinks.protection.outlook.com/?url=http%3A%2F%2Fblog.ffwll.ch%2F&data=04%7C01%7Cchristian.koenig%40amd.com%7C0654a16ea3444271d7c308d9e17bd35d%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637788744226808874%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=Q6soluBglaZLhLszdapaWuUVsqMq5qvJOKiJjO%2B9BTg%3D&reserved=0 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 gabe.freedesktop.org (gabe.freedesktop.org [131.252.210.177]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 36D56C433F5 for ; Thu, 27 Jan 2022 13:24:52 +0000 (UTC) Received: from gabe.freedesktop.org (localhost [127.0.0.1]) by gabe.freedesktop.org (Postfix) with ESMTP id 8302910E1E6; Thu, 27 Jan 2022 13:24:51 +0000 (UTC) Received: from mail-wr1-x42b.google.com (mail-wr1-x42b.google.com [IPv6:2a00:1450:4864:20::42b]) by gabe.freedesktop.org (Postfix) with ESMTPS id F1F3910E179; Thu, 27 Jan 2022 11:44:24 +0000 (UTC) Received: by mail-wr1-x42b.google.com with SMTP id h21so4216808wrb.8; Thu, 27 Jan 2022 03:44:24 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; h=message-id:date:mime-version:user-agent:subject:content-language:to :references:from:in-reply-to:content-transfer-encoding; bh=LdvVnzVP+ambH9LK42MFqP+4gobIMIEV2AtnE2m4+4w=; b=AO2Ubp21T4K3UXrE4ox8RrOuzjwwgrlPkeKckx6Grh9RDZ/sTc4sbmYVdr0jjAzpTw QVQQPpq97aK2jW5XFd7bQnzTVqoBJJj6378shYn3d2cksVOP7/4zyAiwlP+kuExwEhDw XZJrGcwnFl9NeV9+LhDnIPEtlKSJuAh2tjefqYKPD21aILv8pPHehPvOIQyGlgjdFsPj oMzJiea1Io27zOiupzkuaNpJcLwNMgIk7MJ6504aaHlBk3VjA7u9dBrKb0HD/gWe/+pQ RvbtkX3j16vPwnewh6iIwT26ia4TZVLMcK3m4cq6+CdyMByCBzP2UMRi0DkN5ke2sf1n 11WQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:message-id:date:mime-version:user-agent:subject :content-language:to:references:from:in-reply-to :content-transfer-encoding; bh=LdvVnzVP+ambH9LK42MFqP+4gobIMIEV2AtnE2m4+4w=; b=c/DCwdygDLdVV5AHnkh3PTpyChIZYzbPWgAS6ltLMOhTW0sPZy9MKVV/3ncnmUCXHN efKhcPhMGrKLMj0hjJKL0vb3gCvRxiIa5UN4ZWK4GGk9PrVSt7ZbEeaH0MW4wvz29Uvr a1Uu7miDp1VpTp89x6O49A7ypWGn7DVis+Si5ZiE82EyjUuzYhdd3fZ6x2Kn1/yKIYX0 HMHpUVcXZWemX6yNyr+9QGuXN78k/nQ7Arr9SCoPgPOBmPIHyj4PPrnKxEirnh0f2WM4 ZL9qx53LLJVALqBKjBfpCPyZZ6ohuI84PfgbJLKSUdZskg2LTLwDXIbXOoWTF+MV3D6A EFtA== X-Gm-Message-State: AOAM533xy5F+UGYGwl6LmC4zvEMhM+ubwgRtpBHu0fxcDUQEKJOtg/A+ xOP3TXkMIo6pL0NIu+fU9Tw= X-Google-Smtp-Source: ABdhPJx/KQtwKWNuH/SNqZ6CP2qgfsi4lZk1rb4KUhwVnUiM+JCFI4ynUa/ZWR4g7iYh+f08Ls5ePQ== X-Received: by 2002:adf:a410:: with SMTP id d16mr2795659wra.517.1643283863392; Thu, 27 Jan 2022 03:44:23 -0800 (PST) Received: from [192.168.178.21] (p57b0bff8.dip0.t-ipconnect.de. [87.176.191.248]) by smtp.gmail.com with ESMTPSA id a6sm1918382wrx.101.2022.01.27.03.44.22 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 27 Jan 2022 03:44:22 -0800 (PST) Message-ID: <50cf1f2f-3fb2-8abb-7497-dafcd97935f3@gmail.com> Date: Thu, 27 Jan 2022 12:44:21 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.5.0 Content-Language: en-US To: =?UTF-8?Q?Christian_K=c3=b6nig?= , Lucas De Marchi , linaro-mm-sig@lists.linaro.org, intel-gfx@lists.freedesktop.org, linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org, linux-media@vger.kernel.org References: <20220126203702.1784589-1-lucas.demarchi@intel.com> <20220126203702.1784589-3-lucas.demarchi@intel.com> <20220127075728.ygwgorhnrwaocdqv@ldmartin-desk2> <3066c6a7-fc73-d34d-d209-a3ff6818dfb6@amd.com> <20220127093332.wnkd2qy4tvwg5i5l@ldmartin-desk2> <27aed6b1-b465-6a52-2b0a-d748c9798414@amd.com> From: =?UTF-8?Q?Christian_K=c3=b6nig?= In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Mailman-Approved-At: Thu, 27 Jan 2022 13:24:50 +0000 Subject: Re: [Intel-gfx] [Linaro-mm-sig] Re: [PATCH 02/19] dma-buf-map: Add helper to initialize second map X-BeenThere: intel-gfx@lists.freedesktop.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Intel graphics driver community testing & development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: intel-gfx-bounces@lists.freedesktop.org Sender: "Intel-gfx" Am 27.01.22 um 12:16 schrieb Daniel Vetter: > On Thu, Jan 27, 2022 at 11:21:20AM +0100, Christian König wrote: >> Am 27.01.22 um 11:00 schrieb Daniel Vetter: >>> On Thu, Jan 27, 2022 at 01:33:32AM -0800, Lucas De Marchi wrote: >>>> On Thu, Jan 27, 2022 at 09:57:25AM +0100, Daniel Vetter wrote: >>>>> On Thu, Jan 27, 2022 at 09:02:54AM +0100, Christian König wrote: >>>>>> Am 27.01.22 um 08:57 schrieb Lucas De Marchi: >>>>>>> On Thu, Jan 27, 2022 at 08:27:11AM +0100, Christian König wrote: >>>>>>>> Am 26.01.22 um 21:36 schrieb Lucas De Marchi: >>>>>>>>> When dma_buf_map struct is passed around, it's useful to be able to >>>>>>>>> initialize a second map that takes care of reading/writing to an offset >>>>>>>>> of the original map. >>>>>>>>> >>>>>>>>> Add a helper that copies the struct and add the offset to the proper >>>>>>>>> address. >>>>>>>> Well what you propose here can lead to all kind of problems and is >>>>>>>> rather bad design as far as I can see. >>>>>>>> >>>>>>>> The struct dma_buf_map is only to be filled in by the exporter and >>>>>>>> should not be modified in this way by the importer. >>>>>>> humn... not sure if I was  clear. There is no importer and exporter here. >>>>>> Yeah, and exactly that's what I'm pointing out as problem here. >>>>>> >>>>>> You are using the inter driver framework for something internal to the >>>>>> driver. That is an absolutely clear NAK! >>>>>> >>>>>> We could discuss that, but you guys are just sending around patches to do >>>>>> this without any consensus that this is a good idea. >>>>> Uh I suggested this, also we're already using dma_buf_map all over the >>>>> place as a convenient abstraction. So imo that's all fine, it should allow >>>>> drivers to simplify some code where on igpu it's in normal kernel memory >>>>> and on dgpu it's behind some pci bar. >>>>> >>>>> Maybe we should have a better name for that struct (and maybe also a >>>>> better place), but way back when we discussed that bikeshed I didn't come >>>>> up with anything better really. >>>> I suggest iosys_map since it abstracts access to IO and system memory. >>>> >>>>>>> There is a role delegation on filling out and reading a buffer when >>>>>>> that buffer represents a struct layout. >>>>>>> >>>>>>> struct bla { >>>>>>>     int a; >>>>>>>     int b; >>>>>>>     int c; >>>>>>>     struct foo foo; >>>>>>>     struct bar bar; >>>>>>>     int d; >>>>>>> } >>>>>>> >>>>>>> >>>>>>> This implementation allows you to have: >>>>>>> >>>>>>>     fill_foo(struct dma_buf_map *bla_map) { ... } >>>>>>>     fill_bar(struct dma_buf_map *bla_map) { ... } >>>>>>> >>>>>>> and the first thing these do is to make sure the map it's pointing to >>>>>>> is relative to the struct it's supposed to write/read. Otherwise you're >>>>>>> suggesting everything to be relative to struct bla, or to do the same >>>>>>> I'm doing it, but IMO more prone to error: >>>>>>> >>>>>>>     struct dma_buf_map map = *bla_map; >>>>>>>     dma_buf_map_incr(map, offsetof(...)); >>>>> Wrt the issue at hand I think the above is perfectly fine code. The idea >>>>> with dma_buf_map is really that it's just a special pointer, so writing >>>>> the code exactly as pointer code feels best. Unfortunately you cannot make >>>>> them typesafe (because of C), so the code sometimes looks a bit ugly. >>>>> Otherwise we could do stuff like container_of and all that with >>>>> typechecking in the macros. >>>> I had exactly this code above, but after writting quite a few patches >>>> using it, particularly with functions that have to write to 2 maps (see >>>> patch 6 for example), it felt much better to have something to >>>> initialize correctly from the start >>>> >>>> struct dma_buf_map other_map = *bla_map; >>>> /* poor Lucas forgetting dma_buf_map_incr(map, offsetof(...)); */ >>>> >>>> is error prone and hard to debug since you will be reading/writting >>>> from/to another location rather than exploding >>>> >>>> While with the construct below >>>> >>>> other_map; >>>> ... >>>> other_map = INITIALIZER() >>>> >>>> I can rely on the compiler complaining about uninitialized var. And >>>> in most of the cases I can just have this single line in the beggining of the >>>> function when the offset is constant: >>>> >>>> struct dma_buf_map other_map = INITIALIZER(bla_map, offsetof(..)); >>> Hm yeah that's a good point that this allows us to rely on the compiler to >>> check for uninitialized variables. >>> >>> Maybe include the above (with editing, but keeping the examples) in the >>> kerneldoc to explain why/how to use this? With that the concept at least >>> has my >>> >>> Acked-by: Daniel Vetter >>> >>> I'll leave it up to you & Christian to find a prettier color choice for >>> the naming bikeshed. >> There is one major issue remaining with this and that is dma_buf_vunmap(): >> >> void dma_buf_vunmap(struct dma_buf *dmabuf, struct dma_buf_map *map); >> >> Here we expect the original pointer as returned by dma_buf_map(), otherwise >> we vunmap() the wrong area! >> >> For all TTM based driver this doesn't matter since we keep the vmap base >> separately in the BO anyway (IIRC), but we had at least one case where this >> made boom last year. > Yeah but isn't that the same if it's just a void *? > > If you pass the wrong pointer to an unmap function and not exactly what > you go from the map function, then things go boom. This is like > complaining that the following code wont work > > u32 *stuff > > stuff = kmap_local(some_page); > *stuff++ = 0; > *stuff = 1; > kunmap_locak(stuff); > > It's just ... don't do that :-) Also since we pass dma_buf_map by value > and not by pointer anywhere, the risk of this happening is pretty low > since you tend to work on a copy. Same with void * pointers really. > > Now if people start to pass around struct dma_buf_map * as pointers for > anything else than out parameters, then we're screwed. But that's like > passing around void ** for lolz, which is just wrong (except when it's an > out parameter or actually an array of pointers ofc). > > Or I really don't get your concern and you mean something else? No that's pretty much it. It's just that we hide the pointer inside a structure and it is absolutely not obvious to a driver dev that you can't do: dma_buf_vmap(.., &map); dma_buf_map_inr(&map, x); dma_buf_vunmap(.., &map); As bare minimum I strongly suggest that we add some WARN_ONs to the framework to check that the pointer given to dma_buf_vunmap() is at least page aligned. Christian. > -Daniel > > >> Christian. >> >>> -Daniel >>> >>>> Lucas De Marchi >>>> >>>>> -Daniel >>>>> >>>>>>> IMO this construct is worse because at a point in time in the function >>>>>>> the map was pointing to the wrong thing the function was supposed to >>>>>>> read/write. >>>>>>> >>>>>>> It's also useful when the function has double duty, updating a global >>>>>>> part of the struct and a table inside it (see example in patch 6) >>>>>>> >>>>>>> thanks >>>>>>> Lucas De Marchi >>>>> -- >>>>> Daniel Vetter >>>>> Software Engineer, Intel Corporation >>>>> https://nam11.safelinks.protection.outlook.com/?url=http%3A%2F%2Fblog.ffwll.ch%2F&data=04%7C01%7Cchristian.koenig%40amd.com%7C0654a16ea3444271d7c308d9e17bd35d%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637788744226808874%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=Q6soluBglaZLhLszdapaWuUVsqMq5qvJOKiJjO%2B9BTg%3D&reserved=0