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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Spamd-Result: default: False [-2.80 / 50.00]; BAYES_HAM(-3.00)[100.00%]; SUSPICIOUS_RECIPS(1.50)[]; NEURAL_HAM_LONG(-1.00)[-1.000]; NEURAL_HAM_SHORT(-0.20)[-0.999]; MIME_GOOD(-0.10)[text/plain]; RCVD_TLS_ALL(0.00)[]; TAGGED_RCPT(0.00)[]; MIME_TRACE(0.00)[0:+]; RCPT_COUNT_TWELVE(0.00)[17]; ARC_NA(0.00)[]; RCVD_VIA_SMTP_AUTH(0.00)[]; MID_RHS_MATCH_FROM(0.00)[]; FREEMAIL_ENVRCPT(0.00)[gmail.com]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; FROM_HAS_DN(0.00)[]; FREEMAIL_CC(0.00)[lists.freedesktop.org,amd.com,ffwll.ch,linux.intel.com,kernel.org,gmail.com,intel.com]; TO_DN_SOME(0.00)[]; FROM_EQ_ENVFROM(0.00)[]; RCVD_COUNT_TWO(0.00)[2]; TO_MATCH_ENVRCPT_ALL(0.00)[]; DBL_BLOCKED_OPENRESOLVER(0.00)[imap1.dmz-prg2.suse.org:helo, suse.de:email, suse.de:mid, suse.com:url] X-BeenThere: intel-xe@lists.freedesktop.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Intel Xe graphics driver List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: intel-xe-bounces@lists.freedesktop.org Sender: "Intel-xe" Hi Am 22.09.26 um 13:34 schrieb Mario Limonciello: > > > On 9/22/26 03:49, Javier Martinez Canillas wrote: >> On Tue, Sep 22, 2026 at 10:24 AM Thomas Zimmermann >> wrote: >>> >>> Hi Mario >>> >>> Am 08.09.26 um 06:40 schrieb Mario Limonciello: >>>> So far backlights have only been controlled via sysfs. However, >>>> sysfs is >>>> not a proper user-space API for runtime modifications, and never was >>>> intended to provide such. The DRM drivers are now prepared to provide >>>> such a backlight link so user-space can control backlight via DRM >>>> connector properties. This allows us to employ the same >>>> access-management >>>> we use for mode-setting. >>>> >>>> This patch adds a few kernel-internal backlight helpers so we can >>>> modify >>>> backlights from within DRM, a brightness-changed notification, and a >>>> per-device takeover count so that legacy sysfs writes can be inhibited >>>> (-EBUSY) while a luminance-aware DRM client is in control. >>>> >>>> Signed-off-by: David Herrmann >>>> >> >> [...] >> >>>> @@ -150,6 +153,13 @@ static ssize_t bl_power_store(struct device >>>> *dev, struct device_attribute *attr, >>>>        struct backlight_device *bd = to_backlight_device(dev); >>>>        unsigned long power, old_power; >>>> >>>> +     /* A luminance-aware DRM client has taken over this >>>> backlight; the >>>> +      * legacy sysfs interface is disabled until the last such client >>>> +      * goes away. >>>> +      */ >>>> +     if (atomic_read(&bd->drm_takeover) > 0) >>>> +             return -EBUSY; >>>> + >>>>        rc = kstrtoul(buf, 0, &power); >>>>        if (rc) >>>>                return rc; >>>> @@ -214,6 +224,13 @@ static ssize_t brightness_store(struct device >>>> *dev, >>>>        struct backlight_device *bd = to_backlight_device(dev); >>>>        unsigned long brightness; >>>> >>>> +     /* A luminance-aware DRM client has taken over this >>>> backlight; the >>>> +      * legacy sysfs interface is disabled until the last such client >>>> +      * goes away. >>>> +      */ >>>> +     if (atomic_read(&bd->drm_takeover) > 0) >>>> +             return -EBUSY; >>> >>> Can you really do that?  AFAIU sysfs is now a de-facto uapi for >>> backlights. >> >> And regardless if this can be done or not, I belive that would be >> better if this is something that drivers decide to do and just return >> -EBUSY from their struct backlight_ops..update_status. >> >> Ideally the sysfs interface should continue to be work as Thomas said >> and a write could trigger a mode set for example, but I don't know if >> that is feasible to do due locking. >> > > Well the problem ends up being that something can change brightness > behind the compositor's back if you leave sysfs available. > > The whole design here is to put the compositor in control.  For > example if compositor wants to enforce brightness to be a certain > value when certain content is being displayed. And as I mentioned, it might not be that easy. Sysfs has been the entry point to backlight brightness so far AFAIU. Did you consider treating this similar to display hotplugging?  When the user plug a display in or out, the DRM driver sends a message to the compositor in user space. [1]  I think it might be possible to do a similar thing for changes to brightness. [1] https://elixir.bootlin.com/linux/v7.2.6/source/drivers/gpu/drm/drm_probe_helper.c#L747 Best regards Thomas -- -- Thomas Zimmermann Graphics Driver Developer SUSE Software Solutions Germany GmbH Frankenstr. 146, 90461 Nürnberg, Germany, www.suse.com GF: Jochen Jaser, Andrew McDonald, (HRB 36809, AG Nürnberg)