From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from relay1-d.mail.gandi.net (relay1-d.mail.gandi.net [217.70.183.193]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 0D3D84654E for ; Mon, 4 Mar 2024 15:28:36 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.70.183.193 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1709566119; cv=none; b=Cx0seW4SaZSPyMLmUuybdPFcjwaRDOgTrZAsf7A+ic+pEhioxzwtFRq3WNnvVxYC4lqKBKnhMvo1Xt3QdMnIAJf6VtSMDW9+zu4GyIzORxp+dJ1z+iXcZHb3UGkiSz2jCoBWW/63FKdLWO+sKCSnXn1MIntqeV6oBpnmxAybq+U= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1709566119; c=relaxed/simple; bh=3dDcnXbku+Z27ViRbBROCPmutzCnVSXDVXSXJUywoTc=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=HwgiFU8VFpKh3JgO5xunHpnWxRj4c8dz7nMeGM0NNwwjjiZNZT79iVvRnvrK8oBGnyIC4xnBWodDqC5bj/xEsdbDD61NKzroh3HE0P2VUiagXVW1XR+ITIUvrWqcjR8qqvEcKm4mgSxh9jqQTciaTtA9Hg+TQeo2Sckun3xTb+o= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=bootlin.com; spf=pass smtp.mailfrom=bootlin.com; dkim=pass (2048-bit key) header.d=bootlin.com header.i=@bootlin.com header.b=LBp/ntE7; arc=none smtp.client-ip=217.70.183.193 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=bootlin.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=bootlin.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=bootlin.com header.i=@bootlin.com header.b="LBp/ntE7" Received: by mail.gandi.net (Postfix) with ESMTPSA id 1D5C824000A; Mon, 4 Mar 2024 15:28:34 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=bootlin.com; s=gm1; t=1709566115; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=aI3n8d1YGiT9gHLQYdUrdxkKidF5d2P9k5U07rkaydA=; b=LBp/ntE7NGrS55I8dk57MpXv+Qbv9II5moe8TqKu1vg2wRw42ayMkkhiE8TC/PCzdieskq MeH00UrXWnPzBsx7Mm2OrRjMoenGm8hsrnv5qutjdEq9yDbna0r5Pn2Orul2X46qdLqjco qtc/lKFhpRSQ+pZ5F3xGAZAHtgEOxvGk7+BU/QqwOKxmrdulukYGqaT083RWhzFJxVzl6U 0b3gD/C++OMsqfKTU7pxWWkN9wO08AZVexOtu0sAcXZ48BnYg1Yw/M817SyCZxaKCU5O+U i3+ihTaEN42XvqU/P2H5Ikx1VlYgfqEU0YTK5b3A486Wgq5CgzM0Ht5ZYaJJ+g== Date: Mon, 4 Mar 2024 16:28:32 +0100 From: Louis Chauvet To: Pekka Paalanen Cc: Rodrigo Siqueira , Melissa Wen , =?iso-8859-1?Q?Ma=EDra?= Canal , Haneen Mohammed , Daniel Vetter , Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , David Airlie , arthurgrillo@riseup.net, Jonathan Corbet , dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org, jeremie.dautheribes@bootlin.com, miquel.raynal@bootlin.com, thomas.petazzoni@bootlin.com Subject: Re: [PATCH v2 4/9] drm/vkms: Add typedef and documentation for pixel_read and pixel_write functions Message-ID: Mail-Followup-To: Pekka Paalanen , Rodrigo Siqueira , Melissa Wen , =?iso-8859-1?Q?Ma=EDra?= Canal , Haneen Mohammed , Daniel Vetter , Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , David Airlie , arthurgrillo@riseup.net, Jonathan Corbet , dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org, jeremie.dautheribes@bootlin.com, miquel.raynal@bootlin.com, thomas.petazzoni@bootlin.com References: <20240223-yuv-v2-0-aa6be2827bb7@bootlin.com> <20240223-yuv-v2-4-aa6be2827bb7@bootlin.com> <20240226133646.174d3fb2.pekka.paalanen@collabora.com> <20240229110737.27f03450.pekka.paalanen@collabora.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20240229110737.27f03450.pekka.paalanen@collabora.com> X-GND-Sasl: louis.chauvet@bootlin.com Le 29/02/24 - 11:07, Pekka Paalanen a écrit : > On Tue, 27 Feb 2024 16:02:13 +0100 > Louis Chauvet wrote: > > > Le 26/02/24 - 13:36, Pekka Paalanen a écrit : > > > On Fri, 23 Feb 2024 12:37:24 +0100 > > > Louis Chauvet wrote: > > > > > > > Introduce two typedefs: pixel_read_t and pixel_write_t. It allows the > > > > compiler to check if the passed functions take the correct arguments. > > > > Such typedefs will help ensuring consistency across the code base in > > > > case of update of these prototypes. > > > > > > > > Introduce a check around the get_pixel_*_functions to avoid using a > > > > nullptr as a function. > > > > > > > > Document for those typedefs. > > > > > > > > Signed-off-by: Louis Chauvet > > > > --- > > > > drivers/gpu/drm/vkms/vkms_drv.h | 23 +++++++++++++++++++++-- > > > > drivers/gpu/drm/vkms/vkms_formats.c | 8 ++++---- > > > > drivers/gpu/drm/vkms/vkms_formats.h | 4 ++-- > > > > drivers/gpu/drm/vkms/vkms_plane.c | 9 ++++++++- > > > > drivers/gpu/drm/vkms/vkms_writeback.c | 9 ++++++++- > > > > 5 files changed, 43 insertions(+), 10 deletions(-) > > > > > > > > diff --git a/drivers/gpu/drm/vkms/vkms_drv.h b/drivers/gpu/drm/vkms/vkms_drv.h > > > > index 18086423a3a7..886c885c8cf5 100644 > > > > --- a/drivers/gpu/drm/vkms/vkms_drv.h > > > > +++ b/drivers/gpu/drm/vkms/vkms_drv.h > > > > @@ -53,12 +53,31 @@ struct line_buffer { > > > > struct pixel_argb_u16 *pixels; > > > > }; > > > > > > > > +/** > > > > + * typedef pixel_write_t - These functions are used to read a pixel from a > > > > + * `struct pixel_argb_u16*`, convert it in a specific format and write it in the @dst_pixels > > > > + * buffer. > > > > + * > > > > + * @dst_pixel: destination address to write the pixel > > > > + * @in_pixel: pixel to write > > > > + */ > > > > +typedef void (*pixel_write_t)(u8 *dst_pixels, struct pixel_argb_u16 *in_pixel); > > > > > > There are some inconsistencies in pixel_write_t and pixel_read_t. At > > > this point of the series they still operate on a single pixel, but you > > > use dst_pixels and src_pixels, plural. Yet the documentation correctly > > > talks about processing a single pixel. > > > > I will fix this for v4. > > > > > I would also expect the source to be always const, but that's a whole > > > another patch to change. > > > > The v4 will contains a new patch "drm/vkms: Use const pointer for > > pixel_read and pixel_write functions" > > Sounds good! > > > > > [...] > > > > > > diff --git a/drivers/gpu/drm/vkms/vkms_plane.c b/drivers/gpu/drm/vkms/vkms_plane.c > > > > index d5203f531d96..f68b1b03d632 100644 > > > > --- a/drivers/gpu/drm/vkms/vkms_plane.c > > > > +++ b/drivers/gpu/drm/vkms/vkms_plane.c > > > > @@ -106,6 +106,13 @@ static void vkms_plane_atomic_update(struct drm_plane *plane, > > > > return; > > > > > > > > fmt = fb->format->format; > > > > + pixel_read_t pixel_read = get_pixel_read_function(fmt); > > > > + > > > > + if (!pixel_read) { > > > > + DRM_WARN("Pixel format is not supported by VKMS planes. State is inchanged\n"); > > > > > > DRM_WARN() is the kernel equivalent to userspace assert(), right? > > > > For the DRM_WARN it is just a standard prinkt(KERN_WARN, ...) (hidden > > behind drm internal macros). > > My concern here is that does hitting this cause additional breakage of > the UAPI contract? For example, the UAPI contract expects that the old > FB is unreffed and the new FB is reffed by the plane in question. If > this early return causes that FB swap to be skipped, it could cause > secondary unexpected failures or misbehaviour for userspace later. That > could mislead debugging to think that there is a userspace bug. > > Even if you cannot actually read FB due to an internal bug, it would be > good to still apply all the state changes that the UAPI contract > mandates. > > Unless, this is a kernel bug kind of thing which explodes very > verbosely, but DRM_WARN is not that. You are right. In this case I maybe can just create a dummy "pixel_read" which always return black? (The v4 will have it so you can see how it look) This way, I can: - keep the check and avoid null function pointers (and OOPS); - log (WARN, DRM_WARN, DRM_ERROR or whatever suggested by DRM maintainers to warn very loudly); - Apply the requested change from userspace (and don't break the UAPI contract). The resulting behavior will be "the virtual plane is black", which is, I think, not very important. As the primary usage of VKMS is testing, it will just broke all the tests, and a quick look at the kernel logs will show this bug. > > > In that failing the check means an internal invariant was violated, > > > which means a code bug in kernel? > > > > > > Maybe this could be more specific about what invariant was violated? > > > E.g. atomic check should have rejected this attempt already. > > > > I'm not an expert (yet) in DRM, so please correct me: > > When atomic_update is called, the new state is always validated by > > atomic_check before? There is no way to pass something not validated by > > atomic_check to atomic_update? If this is the case, yes, it should be an > > ERROR and not a WARN as an invalid format passed the atomic_check > > verification. > > I only know about the UAPI, I'm not familiar with kernel internals. > > We see that atomic_update returns void, so I think it simply cannot > return errors. To my understanding, atomic_check needs to ensure that > atomic_update cannot fail. There is even UAPI to exercise atomic_check > alone: the atomic commit TEST_ONLY flag. Userspace trusts that flag, and > will not expect an identical atomic commit to fail without TEST_ONLY > when it succeeded with TEST_ONLY. That my understanding of the UAPI/DRM internals too, is my suggestion above sufficient? It will always succeed, no kernel OOPS. > > If so, is this better? > > > > if (!pixel_read) { > > /* > > * This is a bug as the vkms_plane_atomic_check must forbid all unsupported formats. > > */ > > DRM_ERROR("Pixel format %4cc is not supported by VKMS planes.\n", fmt); > > return; > > } > > > > I will put the same code in vkms_writeback.c. > > Maybe maintainers can comment whether even DRM_ERROR is strong enough. > > As for the message, what you wrote in the comment is the most important > part that I'd put in the log. It explains what's going on, while that > "format not supported" is a detail without context. > Is something like this better? /* * This is a bug in vkms_plane_atomic_check. All the supported * format must: * - Be listed in vkms_formats * - Have a pixel_read_line callback */ WARN(true, "Pixel format %4cc is not supported by VKMS planes. This is a kernel bug. Atomic check must forbid this configuration.\n", fmt) > Thanks, > pq [...] Kind regards, Louis Chauvet -- Louis Chauvet, Bootlin Embedded Linux and Kernel engineering https://bootlin.com