linux-kernel - Re: [PATCH v2] usb: core: setup dma_pfn_offset for USB devices and, interfaces

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Date:   Mon, 12 Sep 2016 17:09:19 +0300
From:   Roger Quadros <rogerq@...com>
To:     Alan Stern <stern@...land.harvard.edu>
CC:     <gregkh@...uxfoundation.org>, <ssantosh@...nel.org>,
        <grygorii.strashko@...com>, <linux-usb@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v2] usb: core: setup dma_pfn_offset for USB devices and,
 interfaces

Hi Alan,

On 12/09/16 16:09, Alan Stern wrote:
> On Mon, 12 Sep 2016, Roger Quadros wrote:
> 
>> If dma_pfn_offset is not inherited correctly from the host controller,
>> it might result in sub-optimal configuration as bounce
>> buffer limit might be set to less than optimal level.
>>
>> Consider the mass storage device case.
>> USB storage driver creates a scsi host for the mass storage interface in
>> drivers/usb/storage/usb.c
>> The scsi host parent device is nothing but the the USB interface device.
>> Now, __scsi_init_queue() calls scsi_calculate_bounce_limit() to find out
>> and set the block layer bounce limit.
>> scsi_calculate_bounce_limit() uses dma_max_pfn(host_dev) to get the
>> bounce_limit. host_dev is nothing but the device representing the
>> mass storage interface.
>> If that device doesn't have the right dma_pfn_offset, then dma_max_pfn()
>> is messed up and the bounce buffer limit is wrong.
>>
>> e.g. On Keystone 2 systems, dma_max_pfn() is 0x87FFFF and dma_mask_pfn
>> is 0xFFFFF. Consider a mass storage use case: Without this patch,
>> usb scsi host device (usb-storage) will get a dma_pfn_offset of 0 resulting
>> in a dma_max_pfn() of 0xFFFFF within the scsi layer
>> (scsi_calculate_bounce_limit()).
>> This will result in bounce buffers being unnecessarily used.
>>
>> Hint: On 32-bit ARM platforms dma_max_pfn() = dma_mask_pfn + dma_pfn_offset
>>
>> Signed-off-by: Roger Quadros <rogerq@...com>
>> ---
> 
> How does v2 of this patch differ from v1?

No functional difference.
Just added more explanation in the commit log.

> 
>>  drivers/usb/core/message.c | 6 ++++++
>>  drivers/usb/core/usb.c     | 7 +++++++
>>  2 files changed, 13 insertions(+)
>>
>> diff --git a/drivers/usb/core/message.c b/drivers/usb/core/message.c
>> index 0406a59..66364ea 100644
>> --- a/drivers/usb/core/message.c
>> +++ b/drivers/usb/core/message.c
>> @@ -1863,6 +1863,12 @@ free_interfaces:
>>  		intf->dev.type = &usb_if_device_type;
>>  		intf->dev.groups = usb_interface_groups;
>>  		intf->dev.dma_mask = dev->dev.dma_mask;
>> +		/* Propagate dma_pfn_offset to USB interface.
>> +		 * This is especially required by mass storage interface
>> +		 * which relies on SCSI layer and scsi_calculate_bounce_limit()
>> +		 * to set the bounce buffer limit based on dma_pfn_offset.
>> +		 */
>> +		intf->dev.dma_pfn_offset = dev->dev.dma_pfn_offset;
> 
> I really think this comment isn't necessary.  It seems pretty obvious 
> that if you're copying DMA-related fields from one device to another 
> then you'll want to copy the dma_pfn_offset along with everything else.
> No?
> 
> The explanation in the Changelog is enough, IMO.

Fine with me.

> 
>>  		INIT_WORK(&intf->reset_ws, __usb_queue_reset_device);
>>  		intf->minor = -1;
>>  		device_initialize(&intf->dev);
>> diff --git a/drivers/usb/core/usb.c b/drivers/usb/core/usb.c
>> index 5e80697..7dde93d 100644
>> --- a/drivers/usb/core/usb.c
>> +++ b/drivers/usb/core/usb.c
>> @@ -441,6 +441,13 @@ struct usb_device *usb_alloc_dev(struct usb_device *parent,
>>  	dev->dev.type = &usb_device_type;
>>  	dev->dev.groups = usb_device_groups;
>>  	dev->dev.dma_mask = bus->controller->dma_mask;
>> +	/* Propagate bus controller's dma_pfn_offset to USB device,
>> +	 * as it would be needed to be propagated to USB interfaces.
>> +	 * This is especially required by mass storage interface
>> +	 * which relies on SCSI layer and scsi_calculate_bounce_limit()
>> +	 * to set the bounce buffer limit based on dma_pfn_offset.
>> +	 */
>> +	dev->dev.dma_pfn_offset = bus->controller->dma_pfn_offset;
> 
> Same here.

OK.

> 
>>  	set_dev_node(&dev->dev, dev_to_node(bus->controller));
>>  	dev->state = USB_STATE_ATTACHED;
>>  	dev->lpm_disable_count = 1;
> 
> Alan Stern
> 

cheers,
-roger