linux-kernel - Re: [PATCH] xhci: Set DMA parameters appropriately

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Date:   Fri, 13 Oct 2017 11:48:50 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Marek Szyprowski <m.szyprowski@...sung.com>,
        mathias.nyman@...el.com, gregkh@...uxfoundation.org
Cc:     linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        hch@....de, iommu@...ts.linux-foundation.org
Subject: Re: [PATCH] xhci: Set DMA parameters appropriately

Hi Marek,

On 13/10/17 09:15, Marek Szyprowski wrote:
> Hi Robin,
> 
> On 2017-10-11 15:56, Robin Murphy wrote:
>> xHCI requires that data buffers do not cross 64KB boundaries (and are
>> thus at most 64KB long as well) - whilst xhci_queue_{bulk,isoc}_tx()
>> already split their input buffers into individual TRBs as necessary,
>> it's still a good idea to advertise the limitations via the standard DMA
>> API mechanism, so that most producers like the block layer and the DMA
>> mapping implementations can lay things out correctly to begin with.
>>
>> Signed-off-by: Robin Murphy <robin.murphy@....com>
>> ---
>>   drivers/usb/host/xhci.c | 4 ++++
>>   drivers/usb/host/xhci.h | 3 +++
>>   2 files changed, 7 insertions(+)
>>
>> diff --git a/drivers/usb/host/xhci.c b/drivers/usb/host/xhci.c
>> index 74b4500641c2..1e7e1e3d8c48 100644
>> --- a/drivers/usb/host/xhci.c
>> +++ b/drivers/usb/host/xhci.c
>> @@ -4883,6 +4883,10 @@ int xhci_gen_setup(struct usb_hcd *hcd,
>> xhci_get_quirks_t get_quirks)
>>           dma_set_coherent_mask(dev, DMA_BIT_MASK(32));
>>       }
>>   +    dev->dma_parms = &xhci->dma_parms;
>> +    dma_set_max_seg_size(dev, SZ_64K);
>> +    dma_set_seg_boundary(dev, SZ_64K - 1);
>> +
>>       xhci_dbg(xhci, "Calling HCD init\n");
>>       /* Initialize HCD and host controller data structures. */
>>       retval = xhci_init(hcd);
>> diff --git a/drivers/usb/host/xhci.h b/drivers/usb/host/xhci.h
>> index 7ef69ea0b480..afcae4cc908d 100644
>> --- a/drivers/usb/host/xhci.h
>> +++ b/drivers/usb/host/xhci.h
>> @@ -1767,6 +1767,9 @@ struct xhci_hcd {
>>       struct dma_pool    *small_streams_pool;
>>       struct dma_pool    *medium_streams_pool;
>>   +    /* DMA alignment restrictions */
>> +    struct device_dma_parameters dma_parms;
>> +
>>       /* Host controller watchdog timer structures */
>>       unsigned int        xhc_state;
>>   
> 
> Are you sure that xhci_hcd life time is proper to keep dma_parms? It looks
> that when driver gets removed and xhci_hcd is freed, the dma_parms will
> point to freed memory. Maybe it would make sense to clear dev->dma_parms
> somewhere or definitely change the way dma_parms are allocated?

AFAICS it lives until the last usb_put_hcd() call, which is pretty much
the last thing in the drivers' .remove paths, so it looks to follow the
standard paradigm evidenced by other dma_parms users. Any dangling
pointer after the driver has been unbound will be reinitialised by a
subsequent probe, and anyone using an unbound device for DMA API calls
is very wrong anyway.

> On the other hand 64K is the default segment size if no dma_parms are
> provided, so there is very little value added by this patch.

I prefer to explicitly set the segment size for cleanliness and to
emphasize the difference between "whatever the default value is is fine"
and "we have a real hardware limit of 64K". What really matters here
though is the non-default segment boundary mask - that's the motiviation
for the patch.

Robin.