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From: David Hildenbrand <david@redhat.com>
To: Arnd Bergmann <arnd@arndb.de>
Cc: Linus Walleij <linus.walleij@linaro.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Linux-MM <linux-mm@kvack.org>, Joel Stanley <joel@jms.id.au>,
	David Airlie <airlied@linux.ie>, Daniel Vetter <daniel@ffwll.ch>,
	Andrew Jeffery <andrew@aj.id.au>,
	Lucas Stach <l.stach@pengutronix.de>,
	Russell King <linux+etnaviv@armlinux.org.uk>,
	Christian Gmeiner <christian.gmeiner@gmail.com>,
	Mike Rapoport <rppt@kernel.org>,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>,
	Michal Simek <michal.simek@xilinx.com>,
	Masahiro Yamada <masahiroy@kernel.org>,
	Randy Dunlap <rdunlap@infradead.org>,
	Peter Collingbourne <pcc@google.com>,
	linux-aspeed <linux-aspeed@lists.ozlabs.org>,
	dri-devel <dri-devel@lists.freedesktop.org>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	The etnaviv authors <etnaviv@lists.freedesktop.org>,
	Linux Fbdev development list <linux-fbdev@vger.kernel.org>
Subject: Re: [PATCH v1 2/2] drivers/gpu/drm: don't select DMA_CMA or CMA from aspeed or etnaviv
Date: Fri, 9 Apr 2021 10:09:51 +0200	[thread overview]
Message-ID: <b78f2ec2-3c47-f435-6d5e-fb7330ff3907@redhat.com> (raw)
In-Reply-To: <CAK8P3a3sqZBo8-zye1yiZuD2uMUr0oE_q_QfaK9K54TEgd1Cdw@mail.gmail.com>

On 08.04.21 22:29, Arnd Bergmann wrote:
> On Thu, Apr 8, 2021 at 6:45 PM David Hildenbrand <david@redhat.com> wrote:
>> On 08.04.21 14:49, Linus Walleij wrote:
>>> On Thu, Apr 8, 2021 at 2:01 PM David Hildenbrand <david@redhat.com> wrote:
>>>
>>>>> This is something you could do using a hidden helper symbol like
>>>>>
>>>>> config DRMA_ASPEED_GFX
>>>>>           bool "Aspeed display driver"
>>>>>           select DRM_WANT_CMA
>>>>>
>>>>> config DRM_WANT_CMA
>>>>>           bool
>>>>>           help
>>>>>              Select this from any driver that benefits from CMA being enabled
>>>>>
>>>>> config DMA_CMA
>>>>>           bool "Use CMA helpers for DRM"
>>>>>           default DRM_WANT_CMA
>>>>>
>>>>>             Arnd
>>>>>
>>>>
>>>> That's precisely what I had first, with an additional "WANT_CMA" --  but
>>>> looking at the number of such existing options (I was able to spot 1 !)
>>>
>>> If you do this it probably makes sense to fix a few other drivers
>>> Kconfig in the process. It's not just a problem with your driver.
>>> "my" drivers:
>>>
>>
>> :) I actually wanted to convert them to "depends on DMA_CMA" but ran
>> into recursive dependencies ...
>>
>>> drivers/gpu/drm/mcde/Kconfig
>>> drivers/gpu/drm/pl111/Kconfig
>>> drivers/gpu/drm/tve200/Kconfig
> 
> Right, this is the main problem caused by using 'select' to
> force-enable symbols that other drivers depend on.
> 
> Usually, the answer is to be consistent about the use of 'select'
> and 'depends on', using the former only to enable symbols that
> are hidden, while using 'depends on' for anything that is an
> actual build time dependency.
> 
>> I was assuming these are "real" dependencies. Will it also work without
>> DMA_CMA?
> 
> I think in this case, it is fairly likely to work without DMA_CMA when the
> probe function gets called during a fresh boot, but fairly likely to fail if
> it gets called after the system has run for long enough to fragment the
> free memory.
> 
> The point of DMA_CMA is to make it work reliably.

Right, and even at runtime there is no guarantee that DMA_CMA will do 
anything -- especially if we don't reserve a CMA region (e.g., "cma=X").

So this really sounds like a

"desires DMA_CMA"

and achieving that via an additional symbol or via "default y if ..." 
for DMA_CMA sounds reasonable.

Thanks!

-- 
Thanks,

David / dhildenb


  reply	other threads:[~2021-04-09  8:10 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-08  9:20 [PATCH v1 0/2] drivers: don't select DMA_CMA or CMA David Hildenbrand
2021-04-08  9:20 ` [PATCH v1 1/2] drivers/video/fbdev: don't select DMA_CMA David Hildenbrand
2021-04-08  9:56   ` Mike Rapoport
2021-04-08  9:20 ` [PATCH v1 2/2] drivers/gpu/drm: don't select DMA_CMA or CMA from aspeed or etnaviv David Hildenbrand
2021-04-08  9:56   ` Mike Rapoport
2021-04-08  9:57     ` David Hildenbrand
2021-04-08 10:20   ` Arnd Bergmann
2021-04-08 10:27     ` David Hildenbrand
2021-04-08 10:36       ` David Hildenbrand
2021-04-08 10:44       ` Arnd Bergmann
2021-04-08 11:00         ` David Hildenbrand
2021-04-08 11:44           ` Arnd Bergmann
2021-04-08 12:00             ` David Hildenbrand
2021-04-08 12:12               ` Arnd Bergmann
2021-04-08 12:49               ` Linus Walleij
2021-04-08 13:19                 ` Arnd Bergmann
2021-04-09  8:07                   ` David Hildenbrand
2021-04-08 16:44                 ` David Hildenbrand
2021-04-08 20:29                   ` Arnd Bergmann
2021-04-09  8:09                     ` David Hildenbrand [this message]
2021-04-08 21:45                   ` Linus Walleij
2021-04-08 11:38     ` Daniel Vetter
2021-04-08 16:46       ` Jason Gunthorpe

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