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Subject: Re: use exact allocation for dma coherent memory
To:     Christoph Hellwig <hch@lst.de>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Maxime Ripard <maxime.ripard@bootlin.com>,
        Sean Paul <sean@poorly.run>, David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        Jani Nikula <jani.nikula@linux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@linux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@intel.com>,
        Ian Abbott <abbotti@mev.co.uk>,
        H Hartley Sweeten <hsweeten@visionengravers.com>
Cc:     devel@driverdev.osuosl.org, linux-s390@vger.kernel.org,
        Intel Linux Wireless <linuxwifi@intel.com>,
        linux-rdma@vger.kernel.org, netdev@vger.kernel.org,
        intel-gfx@lists.freedesktop.org, linux-wireless@vger.kernel.org,
        linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org,
        linux-mm@kvack.org, iommu@lists.linux-foundation.org,
        "moderated list:ARM PORT" <linux-arm-kernel@lists.infradead.org>,
        linux-media@vger.kernel.org
References: <20190614134726.3827-1-hch@lst.de> <20190701084833.GA22927@lst.de>
From:   Arend Van Spriel <arend.vanspriel@broadcom.com>
Message-ID: <74eb9d99-6aa6-d1ad-e66d-6cc9c496b2f3@broadcom.com>
Date:   Tue, 2 Jul 2019 11:48:44 +0200
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On 7/1/2019 10:48 AM, Christoph Hellwig wrote:
> On Fri, Jun 14, 2019 at 03:47:10PM +0200, Christoph Hellwig wrote:
>> Switching to a slightly cleaned up alloc_pages_exact is pretty easy,
>> but it turns out that because we didn't filter valid gfp_t flags
>> on the DMA allocator, a bunch of drivers were passing __GFP_COMP
>> to it, which is rather bogus in too many ways to explain.  Arm has
>> been filtering it for a while, but this series instead tries to fix
>> the drivers and warn when __GFP_COMP is passed, which makes it much
>> larger than just adding the functionality.
> 
> Dear driver maintainers,
> 
> can you look over the patches touching your drivers, please?  I'd
> like to get as much as possible of the driver patches into this
> merge window, so that it can you through your maintainer trees.

You made me look ;-) Actually not touching my drivers so I'm off the 
hook. However, I was wondering if drivers could know so I decided to 
look into the DMA-API.txt documentation which currently states:

"""
The flag parameter (dma_alloc_coherent() only) allows the caller to
specify the ``GFP_`` flags (see kmalloc()) for the allocation (the
implementation may choose to ignore flags that affect the location of
the returned memory, like GFP_DMA).
"""

I do expect you are going to change that description as well now that 
you are going to issue a warning on __GFP_COMP. Maybe include that in 
patch 15/16 where you introduce that warning.

Regards,
Arend