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Subject: Re: [PATCH v3] drivers: introduce and use WANT_DMA_CMA for soft
 dependencies on DMA_CMA
To:     Arnd Bergmann <arnd@arndb.de>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Maxime Ripard <mripard@kernel.org>,
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        Daniel Vetter <daniel@ffwll.ch>, Joel Stanley <joel@jms.id.au>,
        Andrew Jeffery <andrew@aj.id.au>,
        Lucas Stach <l.stach@pengutronix.de>,
        Russell King <linux+etnaviv@armlinux.org.uk>,
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        Marek Szyprowski <m.szyprowski@samsung.com>,
        Robin Murphy <robin.murphy@arm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mike Rapoport <rppt@kernel.org>,
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References: <20210409112035.27221-1-david@redhat.com>
 <CAK8P3a31uKNcim0n99=yt3zjZ+LQSw4V4+8PS8daLsBdS0iSYg@mail.gmail.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Message-ID: <53ec94ac-ffe3-d0bc-d081-3489fa03daa1@redhat.com>
Date:   Fri, 9 Apr 2021 15:39:47 +0200
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On 09.04.21 15:35, Arnd Bergmann wrote:
> On Fri, Apr 9, 2021 at 1:21 PM David Hildenbrand <david@redhat.com> wrote:
>>
>> Random drivers should not override a user configuration of core knobs
>> (e.g., CONFIG_DMA_CMA=n). Applicable drivers would like to use DMA_CMA,
>> which depends on CMA, if possible; however, these drivers also have to
>> tolerate if DMA_CMA is not available/functioning, for example, if no CMA
>> area for DMA_CMA use has been setup via "cma=X". In the worst case, the
>> driver cannot do it's job properly in some configurations.
>>
>> For example, commit 63f5677544b3 ("drm/etnaviv: select CMA and DMA_CMA if
>> available") documents
>>          While this is no build dependency, etnaviv will only work correctly
>>          on most systems if CMA and DMA_CMA are enabled. Select both options
>>          if available to avoid users ending up with a non-working GPU due to
>>          a lacking kernel config.
>> So etnaviv really wants to have DMA_CMA, however, can deal with some cases
>> where it is not available.
>>
>> Let's introduce WANT_DMA_CMA and use it in most cases where drivers
>> select CMA/DMA_CMA, or depend on DMA_CMA (in a wrong way via CMA because
>> of recursive dependency issues).
>>
>> We'll assume that any driver that selects DRM_GEM_CMA_HELPER or
>> DRM_KMS_CMA_HELPER would like to use DMA_CMA if possible.
>>
>> With this change, distributions can disable CONFIG_CMA or
>> CONFIG_DMA_CMA, without it silently getting enabled again by random
>> drivers. Also, we'll now automatically try to enabled both, CONFIG_CMA
>> and CONFIG_DMA_CMA if they are unspecified and any driver is around that
>> selects WANT_DMA_CMA -- also implicitly via DRM_GEM_CMA_HELPER or
>> DRM_KMS_CMA_HELPER.
>>
>> For example, if any driver selects WANT_DMA_CMA and we do a
>> "make olddefconfig":
>>
>> 1. With "# CONFIG_CMA is not set" and no specification of
>>     "CONFIG_DMA_CMA"
>>
>> -> CONFIG_DMA_CMA won't be part of .config
>>
>> 2. With no specification of CONFIG_CMA or CONFIG_DMA_CMA
>>
>> Contiguous Memory Allocator (CMA) [Y/n/?] (NEW)
>> DMA Contiguous Memory Allocator (DMA_CMA) [Y/n/?] (NEW)
>>
>> 3. With "# CONFIG_CMA is not set" and "# CONFIG_DMA_CMA is not set"
>>
>> -> CONFIG_DMA_CMA will be removed from .config
>>
>> Note: drivers/remoteproc seems to be special; commit c51e882cd711
>> ("remoteproc/davinci: Update Kconfig to depend on DMA_CMA") explains that
>> there is a real dependency to DMA_CMA for it to work; leave that dependency
>> in place and don't convert it to a soft dependency.
> 
> I don't think this dependency is fundamentally different from the others,
> though davinci machines tend to have less memory than a lot of the
> other machines, so it's more likely to fail without CMA.
> 

I was also unsure - and Lucas had similar thoughts. If you want, I can 
send a v4 also taking care of this.

Thanks!

> Regardless of this,
> 
> Reviewed-by: Arnd Bergmann <arnd@arndb.de>
> 


-- 
Thanks,

David / dhildenb