linux-kernel - Re: [PATCH v3 09/24] wfx: add hwio.c/hwio.h

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Date:   Wed, 23 Dec 2020 09:01:33 +0100
From:   Jérôme Pouiller <jerome.pouiller@...abs.com>
To:     Kalle Valo <kvalo@...eaurora.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     devel@...verdev.osuosl.org, devicetree@...r.kernel.org,
        Ulf Hansson <ulf.hansson@...aro.org>, netdev@...r.kernel.org,
        linux-wireless@...r.kernel.org, linux-kernel@...r.kernel.org,
        Rob Herring <robh+dt@...nel.org>, linux-mmc@...r.kernel.org,
        Pali Rohár <pali@...nel.org>,
        "David S . Miller" <davem@...emloft.net>
Subject: Re: [PATCH v3 09/24] wfx: add hwio.c/hwio.h

On Tuesday 22 December 2020 16:27:01 CET Greg Kroah-Hartman wrote:
> On Tue, Dec 22, 2020 at 05:10:11PM +0200, Kalle Valo wrote:
> > Jerome Pouiller <Jerome.Pouiller@...abs.com> writes:
> >
> > > +/*
> > > + * Internal helpers.
> > > + *
> > > + * About CONFIG_VMAP_STACK:
> > > + * When CONFIG_VMAP_STACK is enabled, it is not possible to run DMA on stack
> > > + * allocated data. Functions below that work with registers (aka functions
> > > + * ending with "32") automatically reallocate buffers with kmalloc. However,
> > > + * functions that work with arbitrary length buffers let's caller to handle
> > > + * memory location. In doubt, enable CONFIG_DEBUG_SG to detect badly located
> > > + * buffer.
> > > + */
> >
> > This sounds very hacky to me, I have understood that you should never
> > use stack with DMA.
> 
> You should never do that because some platforms do not support it, so no
> driver should ever try to do that as they do not know what platform they
> are running on.

Just to be curious, why these platforms don't support DMA in a stack
allocated area? If the memory is contiguous (= not vmalloced), correctly
aligned and in the first 4GB of physical memory, it should be sufficient,
shouldn't? 

-- 
Jérôme Pouiller