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

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Date:   Tue, 17 Oct 2017 14:05:48 +0200
From:   Marek Szyprowski <m.szyprowski@...sung.com>
To:     Robin Murphy <robin.murphy@....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 Robin,

On 2017-10-13 12:48, Robin Murphy wrote:
> On 13/10/17 09:15, Marek Szyprowski wrote:
>> 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.

Okay.

>> 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.

Okay. What about other type of USB HCD (EHCI, OHCI, UHCI)?

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland