linux-kernel - Re: [PATCH] drm: msm: Fix build when legacy fbdev support isn't set

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Date:	Thu, 05 Mar 2015 15:36:27 +0530
From:	Archit Taneja <architt@...eaurora.org>
To:	Rob Clark <robdclark@...il.com>,
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
	linux-arm-msm <linux-arm-msm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm: msm: Fix build when legacy fbdev support isn't set


On 02/23/2015 09:09 PM, Daniel Vetter wrote:
> On Mon, Feb 23, 2015 at 10:03:21AM -0500, Rob Clark wrote:
>> On Mon, Feb 23, 2015 at 9:09 AM, Daniel Vetter <daniel@...ll.ch> wrote:
>>> On Mon, Feb 23, 2015 at 08:33:36AM -0500, Rob Clark wrote:
>>>> On Mon, Feb 23, 2015 at 5:29 AM, Archit Taneja <architt@...eaurora.org> wrote:
>>>>> The DRM_KMS_FB_HELPER config is selected only when DRM_MSM_FBDEV config is
>>>>> selected. The driver accesses drm_fb_helper_* functions even when legacy fbdev
>>>>> support is disabled in msm. Wrap around these functions with #ifdef checks to
>>>>> prevent build break.
>>>>
>>>> hmm, perhaps rather than solving this in each driver, we should do
>>>> some stub versions of those fb-helper fxns?
>>>>
>>>> There are at least one or two other drivers that can build without
>>>> fbdev, and I guess more going forward..
>>>
>>> It's not quite that easy since you also have to start/stop the vt
>>> subsystem at the right point in time in your own driver. See
>>> intel_fbdev_set_suspend. If you don't do that there's no synchronization
>>> between fbcon shutting down/resuming and your driver, which in the best
>>> case means fbcon does some writes to nowhere and worst case means your
>>> chip dies (mmio to offline chip blocks) or writes go to somewhere random
>>> in system memory (iommu contains some stale ptes since not yet fully
>>> restored, more an issue with hibernate).
>>
>> I guess I don't fully follow the vt/fbcon interaction if there is no
>> fbdev driver...  but then again I don't have vesafb/efifb to contend
>> with, so I'm assuming something to do with that..
>
> It's the other way round: There's interaction when we have fbdev enabled
> beyond just calling a few fbdev helper functions. And we should compile
> that out too since the console_lock is way too evil ;-)
>
> Only with these additional #ifdefs is i915 completely console_lock free if
> you disable i915 fbdev support. Just stubbing out the fbdev helper
> functions is not enough.
>
>>> And because the console_lock is massively contended we do that in a async
>>> worker in i915.
>>>
>>> But anyway I agree it would still simply drivers quite a bit if we'd have
>>> support for dummy fb helpers in the core, maybe with an explicit Kconfig.
>>> Then drivers could switch to using that for the additional #ifdef (like
>>> the vt stuff i915 does) and otherwise rely upon dummy static inline. That
>>> would give us fbdev-less support for most drivers for free, which is kinda
>>> neat.
>>
>> I guess at least for all the arm drivers, life without fbdev doesn't
>> have these extra complications, so at least they could use stubs..
>
> Does the problem sound more tricky with the above clarification? If you
> don't do the fb_set_suspend call then I expect you'll have some
> interesting problems.
>
>> Plus, I kind of expect phone/tablet/chromebook type stuff would lead
>> the charge into an fbdev-less world..
>
> Yeah, that's another reason to support fbdev-less in the helpers instead
> of each driver.

I was trying to create a patch with the idea above. This works well if 
we want the kernel to support only one DRM driver. If the kernel 
supports multiple platforms and one DRM driver sets its config to enable 
legacy fbdev and another doesn't, we still end up building the fbdev 
helper funcs. Drivers built without legacy fbdev would need to be very 
strict(check for priv->fbdev not NULL) to prevent calling them.

The fb cma helper also adds to the difficulties. The cma helper seems to 
have some functions that provide legacy fbdev support and others that 
allow allocation of drm_framebuffers and gem objects. We'd need to be 
careful about our stub functions not messing up the drivers using the fb 
cma helpers.

Rather than creating fb helper stub functions, maybe we could help each 
drm driver create two variants of functions needed by drm core(like 
output_poll_changed and dev_lastclose), one variant supporting legacy 
fbdev, and the other not?

Archit

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