linux-kernel - Re: use exact allocation for dma coherent memory

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Date:   Tue, 2 Jul 2019 11:48:44 +0200
From:   Arend Van Spriel <arend.vanspriel@...adcom.com>
To:     Christoph Hellwig <hch@....de>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        Sean Paul <sean@...rly.run>, David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Ian Abbott <abbotti@....co.uk>,
        H Hartley Sweeten <hsweeten@...ionengravers.com>
Cc:     devel@...verdev.osuosl.org, linux-s390@...r.kernel.org,
        Intel Linux Wireless <linuxwifi@...el.com>,
        linux-rdma@...r.kernel.org, netdev@...r.kernel.org,
        intel-gfx@...ts.freedesktop.org, linux-wireless@...r.kernel.org,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-mm@...ck.org, iommu@...ts.linux-foundation.org,
        "moderated list:ARM PORT" <linux-arm-kernel@...ts.infradead.org>,
        linux-media@...r.kernel.org
Subject: Re: use exact allocation for dma coherent memory



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