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 4BBE1C3ABBE for ; Thu, 8 May 2025 14:27:29 +0000 (UTC) Received: from gabe.freedesktop.org (localhost [127.0.0.1]) by gabe.freedesktop.org (Postfix) with ESMTP id B112010E1D9; Thu, 8 May 2025 14:27:28 +0000 (UTC) Authentication-Results: gabe.freedesktop.org; dkim=pass (2048-bit key; unprotected) header.d=intel.com header.i=@intel.com header.b="XVp5Fvc/"; dkim-atps=neutral Received: from mgamail.intel.com (mgamail.intel.com [198.175.65.11]) by gabe.freedesktop.org (Postfix) with ESMTPS id 65B6610E3B0 for ; Thu, 8 May 2025 14:27:27 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1746714448; x=1778250448; h=from:to:cc:subject:in-reply-to:references:date: message-id:mime-version:content-transfer-encoding; bh=kbAM4QD7gVOvVtJMaQ5P/9fwrgos1J3zcUB/8eme5jI=; b=XVp5Fvc/wFN5cyodnVszXzba8ikfVs5ge/ggtWGF6nDtBj2bXWLo9bFa S5S+UzCSeLMfEboABS5ZSoJpZHlJk+1VzkpqBiorfBNd14NKRzjmTME4L 2GDJJ91IwnEjJe82YhnPT9nzhcKvxwNAHmuYeSGp9ISERSnoYgehI2Cmr kXMV7EGotVjYfGMSAUhefDrSrsVCNJfvjYiVzJcXM/3432+/eFFVh4rnV Nnne13wsVYD8bNL6oIFAcd4pgJ4kB0iqNFDmG3aTNdO//0Lvo493+fWCC CARGYDoT8IvV+/or+p3wU2K950XoJ/zTM6kvtfXej0udOwJJ0rUh8Rb1V g==; X-CSE-ConnectionGUID: idHPvXPBRey8fxbiyOkCtA== X-CSE-MsgGUID: +JQ7zWwIQRK/Mn+WweLiAQ== X-IronPort-AV: E=McAfee;i="6700,10204,11427"; a="58710856" X-IronPort-AV: E=Sophos;i="6.15,272,1739865600"; d="scan'208";a="58710856" Received: from orviesa009.jf.intel.com ([10.64.159.149]) by orvoesa103.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 08 May 2025 07:27:27 -0700 X-CSE-ConnectionGUID: wgTvTDRjQcKkVioN/GNJDg== X-CSE-MsgGUID: 1JmDS+WjTPKIyxZFg7LG5Q== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.15,272,1739865600"; d="scan'208";a="136021864" Received: from unknown (HELO localhost) ([10.237.66.160]) by orviesa009-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 08 May 2025 07:27:25 -0700 From: Jani Nikula To: Anusha Srivatsa , Maxime Ripard Cc: Neil Armstrong , Jessica Zhang , Maarten Lankhorst , Thomas Zimmermann , David Airlie , Simona Vetter , dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org, Luca Ceresoli Subject: Re: [PATCH v4 2/4] drm/panel: Add refcount support In-Reply-To: Organization: Intel Finland Oy - BIC 0357606-4 - Westendinkatu 7, 02160 Espoo References: <20250331-b4-panel-refcounting-v4-0-dad50c60c6c9@redhat.com> <20250331-b4-panel-refcounting-v4-2-dad50c60c6c9@redhat.com> <87y0vkw8ll.fsf@intel.com> <20250429-benign-sidewinder-of-defense-6dd4d8@houat> <87o6wfwcef.fsf@intel.com> <20250505-slim-bizarre-marten-a674ac@houat> Date: Thu, 08 May 2025 17:27:21 +0300 Message-ID: <874ixvtbxy.fsf@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-BeenThere: dri-devel@lists.freedesktop.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Direct Rendering Infrastructure - Development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dri-devel-bounces@lists.freedesktop.org Sender: "dri-devel" On Mon, 05 May 2025, Anusha Srivatsa wrote: > On Mon, May 5, 2025 at 2:54=E2=80=AFAM Maxime Ripard = wrote: > >> Hi Jani, >> >> On Tue, Apr 29, 2025 at 12:22:00PM +0300, Jani Nikula wrote: >> > On Tue, 29 Apr 2025, Maxime Ripard wrote: >> > > Hi Jani, >> > > >> > > On Mon, Apr 28, 2025 at 07:31:50PM +0300, Jani Nikula wrote: >> > >> On Mon, 31 Mar 2025, Anusha Srivatsa wrote: >> > >> > Allocate panel via reference counting. Add _get() and _put() help= er >> > >> > functions to ensure panel allocations are refcounted. Avoid use >> after >> > >> > free by ensuring panel pointer is valid and can be usable till the >> last >> > >> > reference is put. >> > >> > >> > >> > Reviewed-by: Luca Ceresoli >> > >> > Reviewed-by: Maxime Ripard >> > >> > Signed-off-by: Anusha Srivatsa >> > >> > >> > >> > --- >> > >> > v4: Add refcounting documentation in this patch (Maxime) >> > >> > >> > >> > v3: Add include in this patch (Luca) >> > >> > >> > >> > v2: Export drm_panel_put/get() (Maxime) >> > >> > - Change commit log with better workding (Luca, Maxime) >> > >> > - Change drm_panel_put() to return void (Luca) >> > >> > - Code Cleanups - add return in documentation, replace bridge to >> > >> > panel (Luca) >> > >> > --- >> > >> > drivers/gpu/drm/drm_panel.c | 64 >> ++++++++++++++++++++++++++++++++++++++++++++- >> > >> > include/drm/drm_panel.h | 19 ++++++++++++++ >> > >> > 2 files changed, 82 insertions(+), 1 deletion(-) >> > >> > >> > >> > diff --git a/drivers/gpu/drm/drm_panel.c >> b/drivers/gpu/drm/drm_panel.c >> > >> > index >> bdeab5710ee324dc1742fbc77582250960556308..7b17531d85a4dc3031709919564d2e= 4d8332f748 >> 100644 >> > >> > --- a/drivers/gpu/drm/drm_panel.c >> > >> > +++ b/drivers/gpu/drm/drm_panel.c >> > >> > @@ -355,24 +355,86 @@ struct drm_panel *of_drm_find_panel(const >> struct device_node *np) >> > >> > } >> > >> > EXPORT_SYMBOL(of_drm_find_panel); >> > >> > >> > >> > +static void __drm_panel_free(struct kref *kref) >> > >> > +{ >> > >> > + struct drm_panel *panel =3D container_of(kref, struct >> drm_panel, refcount); >> > >> > + >> > >> > + kfree(panel->container); >> > >> > +} >> > >> > + >> > >> > +/** >> > >> > + * drm_panel_get - Acquire a panel reference >> > >> > + * @panel: DRM panel >> > >> > + * >> > >> > + * This function increments the panel's refcount. >> > >> > + * Returns: >> > >> > + * Pointer to @panel >> > >> > + */ >> > >> > +struct drm_panel *drm_panel_get(struct drm_panel *panel) >> > >> > +{ >> > >> > + if (!panel) >> > >> > + return panel; >> > >> > + >> > >> > + kref_get(&panel->refcount); >> > >> > + >> > >> > + return panel; >> > >> > +} >> > >> > +EXPORT_SYMBOL(drm_panel_get); >> > >> > + >> > >> > +/** >> > >> > + * drm_panel_put - Release a panel reference >> > >> > + * @panel: DRM panel >> > >> > + * >> > >> > + * This function decrements the panel's reference count and frees >> the >> > >> > + * object if the reference count drops to zero. >> > >> > + */ >> > >> > +void drm_panel_put(struct drm_panel *panel) >> > >> > +{ >> > >> > + if (panel) >> > >> > + kref_put(&panel->refcount, __drm_panel_free); >> > >> > +} >> > >> > +EXPORT_SYMBOL(drm_panel_put); >> > >> > + >> > >> > +/** >> > >> > + * drm_panel_put_void - wrapper to drm_panel_put() taking a void >> pointer >> > >> > + * >> > >> > + * @data: pointer to @struct drm_panel, cast to a void pointer >> > >> > + * >> > >> > + * Wrapper of drm_panel_put() to be used when a function taking a >> void >> > >> > + * pointer is needed, for example as a devm action. >> > >> > + */ >> > >> > +static void drm_panel_put_void(void *data) >> > >> > +{ >> > >> > + struct drm_panel *panel =3D (struct drm_panel *)data; >> > >> > + >> > >> > + drm_panel_put(panel); >> > >> > +} >> > >> > + >> > >> > void *__devm_drm_panel_alloc(struct device *dev, size_t size, >> size_t offset, >> > >> > const struct drm_panel_funcs *funcs, >> > >> > int connector_type) >> > >> > { >> > >> > void *container; >> > >> > struct drm_panel *panel; >> > >> > + int err; >> > >> > >> > >> > if (!funcs) { >> > >> > dev_warn(dev, "Missing funcs pointer\n"); >> > >> > return ERR_PTR(-EINVAL); >> > >> > } >> > >> > >> > >> > - container =3D devm_kzalloc(dev, size, GFP_KERNEL); >> > >> > + container =3D kzalloc(size, GFP_KERNEL); >> > >> > if (!container) >> > >> > return ERR_PTR(-ENOMEM); >> > >> > >> > >> > panel =3D container + offset; >> > >> > + panel->container =3D container; >> > >> > panel->funcs =3D funcs; >> > >> > + kref_init(&panel->refcount); >> > >> >> > >> Hi Anusha, this should be done in drm_panel_init() instead. >> > >> >> > >> There are many users of drm_panel that don't use >> devm_drm_panel_alloc() >> > >> but allocate separately, and call drm_panel_init() only. >> > > >> > > That wouldn't really work, because then drivers would have allocated >> the >> > > panel with devm_kzalloc and thus the structure would be freed when t= he >> > > device is removed, no matter the reference counting state. >> > > >> > >> They'll all have refcount set to 0 instead of 1 like kref_init() do= es. >> > >> >> > >> This means all subsequent get/put pairs on such panels will lead to >> > >> __drm_panel_free() being called! But through a lucky coincidence, t= hat >> > >> will be a nop because panel->container is also not initialized... >> > >> >> > >> I'm sorry to say, the drm refcounting interface is quite broken for >> such >> > >> use cases. >> > > >> > > The plan is to convert all panel drivers to that function, and Anusha >> > > already sent series to do. It still needs a bit of work, but it shou= ld >> > > land soon-ish. >> > > >> > > For the transitional period though, it's not clear to me what you th= ink >> > > is broken at the moment, and / or what should be fixed. >> > > >> > > Would you prefer an explicit check on container not being 0, with a >> > > comment? >> > >> > I'm looking at what it would take to add drm_panel support to i915 so >> > that you could have drm_panel_followers on it. There are gaps of cours= e, >> > but initially it would mean allocating and freeing drm_panel ourselves, >> > not via devm_drm_panel_alloc() nor devm_kzalloc(), because none of the >> > other stuff is allocated that way. drm_panel would just sit as a >> > sub-struct inside struct intel_panel, which is a sub-struct of struct >> > intel_connector, which has its own allocation... >> >> I'm not entirely sure why you would need to allocate it from i915? The >> drm_panel structure is only meant to be allocated by panel drivers, and >> afaik no panel interface controller is allocating it. I'm looking into a use case involving drm_panel_follower, which requires a drm_panel. I don't really need any of the other stuff in drm_panel. And basically you'd have one drm_panel per connector that is connected to a panel, within the same driver. >> > But basically in its current state, the refcounting would not be >> > reliable for that use case. I guess with panel->container being NULL >> > nothing happens, but the idea that the refcount drops back to 0 after a >> > get/put is a bit scary. >> > >> > Anyway, I think there should be no harm in moving the kref init to >> > drm_panel_init(), right? >> >> I mean, there is because the plan so far was to remove drm_panel_init() = :) The problem with that is that it forces a certain type of allocation of drm_panel. devm_drm_panel_alloc() allows embedding drm_panel inside another struct, but that's inflexible for our use case, where we'd probably like to embed it inside something that's already allocated as part of something else. I mean devm_drm_panel_alloc() is great, but please don't make its use mandatory! > Jani, > the series that converts all drivers to use the new API: > https://patchwork.freedesktop.org/series/147082/ > https://patchwork.freedesktop.org/series/147157/ > https://patchwork.freedesktop.org/series/147246/ > > not landed yet but these are WIP. Still trying to understand your point > though... not sure what is broken. Nothing upstream is broken per se, but if you allocated drm_panel directly yourself, initialized it with drm_panel_init(), its refcount would initially be 0, not 1, and each subsequent get/put on it would call __drm_panel_free(). Even that doesn't break stuff, because by luck panel->container is also NULL in this case. BR, Jani. --=20 Jani Nikula, Intel