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

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Date:   Tue, 12 Oct 2021 15:50:48 +0200
From:   Cornelia Huck <cohuck@...hat.com>
To:     Halil Pasic <pasic@...ux.ibm.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 Mon, Oct 11 2021, Halil Pasic <pasic@...ux.ibm.com> wrote:

> On Mon, 11 Oct 2021 16:33:45 +0200
> Cornelia Huck <cohuck@...hat.com> wrote:
>
>> On Mon, Oct 11 2021, Pierre Morel <pmorel@...ux.ibm.com> wrote:
>> 
>> > On 10/11/21 1:59 PM, Halil Pasic wrote:  
>> >> diff --git a/drivers/s390/cio/device_ops.c b/drivers/s390/cio/device_ops.c
>> >> index 0fe7b2f2e7f5..c533d1dadc6b 100644
>> >> --- a/drivers/s390/cio/device_ops.c
>> >> +++ b/drivers/s390/cio/device_ops.c
>> >> @@ -825,13 +825,23 @@ EXPORT_SYMBOL_GPL(ccw_device_get_chid);
>> >>    */
>> >>   void *ccw_device_dma_zalloc(struct ccw_device *cdev, size_t size)
>> >>   {
>> >> -	return cio_gp_dma_zalloc(cdev->private->dma_pool, &cdev->dev, size);
>> >> +	void *addr;
>> >> +
>> >> +	if (!get_device(&cdev->dev))
>> >> +		return NULL;
>> >> +	addr = cio_gp_dma_zalloc(cdev->private->dma_pool, &cdev->dev, size);
>> >> +	if (IS_ERR_OR_NULL(addr))  
>> >
>> > I can be wrong but it seems that only dma_alloc_coherent() used in 
>> > cio_gp_dma_zalloc() report an error but the error is ignored and used as 
>> > a valid pointer.  
>> 
>> Hm, I thought dma_alloc_coherent() returned either NULL or a valid
>> address?
>
> Yes, that is what is documented.
>
>> 
>> >
>> > So shouldn't we modify this function and just test for a NULL address here?  
>> 
>> 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
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.