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From: Arnd Bergmann <arnd@arndb.de>
To: Roger Quadros <rogerq@ti.com>
Cc: gregkh@linuxfoundation.org, stern@rowland.harvard.edu,
	ssantosh@kernel.org, grygorii.strashko@ti.com,
	linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4] usb: core: setup dma_pfn_offset for USB devices and, interfaces
Date: Tue, 13 Sep 2016 12:36:43 +0200	[thread overview]
Message-ID: <4403953.WnstcHvg1n@wuerfel> (raw)
In-Reply-To: <92772fe0-5c3d-b2f0-6ab3-0504f29de7e4@ti.com>

On Tuesday, September 13, 2016 11:16:03 AM CEST 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@ti.com>

Acked-by: Arnd Bergmann <arnd@arndb.de>

  reply	other threads:[~2016-09-13 10:37 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-08-17 12:35 [PATCH] usb: core: setup dma_pfn_offset for USB devices Roger Quadros
2016-09-12 11:26 ` [PATCH v2] usb: core: setup dma_pfn_offset for USB devices and, interfaces Roger Quadros
2016-09-12 13:09   ` Alan Stern
2016-09-12 14:09     ` Roger Quadros
2016-09-12 14:34     ` Arnd Bergmann
2016-09-12 18:16       ` Alan Stern
2016-09-12 14:14   ` [PATCH v3] " Roger Quadros
2016-09-13  8:16     ` [PATCH v4] " Roger Quadros
2016-09-13 10:36       ` Arnd Bergmann [this message]
2016-09-13 13:12         ` Alan Stern

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