linux-kernel - Re: [RFC PATCH 1/1] s390/cio: make ccw_device_dma

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Message-ID: <20211013003714.1c411f0b.pasic@linux.ibm.com>
Date:   Wed, 13 Oct 2021 00:37:14 +0200
From:   Halil Pasic <pasic@...ux.ibm.com>
To:     Cornelia Huck <cohuck@...hat.com>
Cc:     Pierre Morel <pmorel@...ux.ibm.com>,
        Vineeth Vijayan <vneethv@...ux.ibm.com>,
        Peter Oberparleiter <oberpar@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Michael Mueller <mimu@...ux.ibm.com>,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org, bfu@...hat.com,
        Halil Pasic <pasic@...ux.ibm.com>
Subject: Re: [RFC PATCH 1/1] s390/cio: make ccw_device_dma_* more robust

On Tue, 12 Oct 2021 15:50:48 +0200
Cornelia Huck <cohuck@...hat.com> wrote:

> >> If I read cio_gp_dma_zalloc() correctly, we either get NULL or a valid
> >> address, so yes.
> >>   
> >
> > I don't think the extra care will hurt us too badly. I prefer to keep
> > the IS_ERR_OR_NULL() check because it needs less domain specific
> > knowledge to be understood, and because it is more robust.  
> 
> It feels weird, though -- I'd rather have a comment that tells me

This way the change feels simpler and safer to me. I believe I explained
the why above. But if you insist I can change it. I double checked the
cio_gp_dma_zalloc() code, and more or less the code called by it. So
now I don't feel uncomfortable with the simpler check.

On the other hand, I'm not very happy doing changes solely based on
somebody's feelings. It would feel much more comfortable with a reason
based discussion.

One reason to change this to a simple NULL check, is that the
IS_ERR_OR_NULL() check could upset the reader of the client code,
which only checks for NULL.

On the other hand I do believe we have some risk of lumping together
different errors here. E.g. dma_pool is NULL or dma ops are not set up
properly. Currently we would communicate that kind of a problem as
-ENOMEM, which wouldn't be a great match. But since dma_alloc_coherent()
returns either NULL or a valid pointer, and furthermore this looks like
a common thing in all the mm-api, I decided to be inline with that.

TLDR; If you insist, I will change this to a simple null pointer check.

> exactly what cio_gp_dma_zalloc() is supposed to return; I would have
> expected that a _zalloc function always gives me a valid pointer or
> NULL.

I don't think we have such a comment for dma_alloc_coherent() or even
kmalloc(). I agree, it would be nice to have this behavior documented
in the apidoc all over the place. But IMHO that is a different issue.

Regards,
Halil