[U-Boot] [PATCH 4/6] usb:udc:samsung: Zero copy approach for data passed to Samsung's UDC driver

[Lukasz Majewski]

Hi Marek,

> On Monday, February 03, 2014 at 12:06:59 PM, Lukasz Majewski wrote:
> > Hi Marek,
> > 
> > > On Saturday, February 01, 2014 at 12:05:29 PM, Lukasz Majewski
> > > wrote:
> > > > On Sat, 1 Feb 2014 03:55:20 +0100
> > > > 
> > > > Marek Vasut <marex at denx.de> wrote:
> > > > > On Friday, January 31, 2014 at 01:16:27 PM, Lukasz Majewski
> > > > > wrote:
> > > > > > The Samsung's UDC driver is not anymore copying data from
> > > > > > USB requests to data aligned internal buffers. Now it works
> > > > > > directly in data allocated in the upper layers like UMS,
> > > > > > DFU, THOR.
> > > > > > 
> > > > > > This change is possible since those gadgets now take care to
> > > > > > allocate buffers aligned to cache line
> > > > > > (CONFIG_SYS_CACHELINE_SIZE ).
> > > > > > 
> > > > > > Previously the UDC needed to copy this data to internal
> > > > > > aligned buffer to prevent from unaligned access exceptions.
> > > > > > 
> > > > > > Test condition
> > > > > > - test HW + measurement: Trats - Exynos4210 rev.1
> > > > > > - test HW Trats2 - Exynos4412 rev.1
> > > > > > 400 MiB compressed rootfs image download with `thor 0 mmc 0`
> > > > > > 
> > > > > > Measurement:
> > > > > > Transmission speed: 27.04 MiB/s
> > > > > > 
> > > > > > Change-Id: I1df1fbafc72ec703f0367ddee3fedf3a3f5523ed
> > > > > > Signed-off-by: Lukasz Majewski <l.majewski at samsung.com>
> > > > > > Cc: Marek Vasut <marex at denx.de>
> > > > > 
> > > > > You should use ROUND_UP(), not ROUND() throughout the patch.
> > > > > Otherwise you might fail to flush/invalidate the last little
> > > > > bit of data in some cacheline.
> > > > 
> > > > I might overlooked something, so please correct me if needed.
> > > > 
> > > > I allocate buffers in gadgets which are aligned to cache line
> > > > with starting address and its size is a multiplication of cache
> > > > line size (so I will not trash data allocated next to it when I
> > > > invalidate cache).
> > > > 
> > > > In the code I'm using ROUND to invalidate/flush more data than
> > > > needed (ROUND(176, 32) = 192). I'm prepared for this since
> > > > buffer in gadget is properly allocated (with
> > > > DEFINE_CACHE_ALIGN_BUFFER() which uses roundup() internally).
> > > 
> > > The problem is in case you receive buffer which is aligned to
> > > cacheline with it's start, but is [(k * cacheline_size) +
> > > (cacheline_size / 2) - 1] big. I think it's unlikely, but if this
> > > happens, you will get corruption, right ?
> > 
> > Let's suppose, that I will receive 2063B = [(64 * 32) + 16 -1] from
> > UDC. If the passed buffer was exactly 2063 B in size, then we would
> > have here a data corruption.
> > 
> > However this situation will not happen
> 
> _Should_ not happen ... I am absolutelly positive someone will be
> bitten by such assumption. I think this assumption about buffer
> alignment should really be documented somewhere.

I will add verbose commit message for this.

> 
> > since the buffer at gadget is
> > allocated with DEFINE_CACHE_ALIGN_BUFFER() or is an aligned
> > multiplication of cache line size (like 1MiB).
> > 
> > I think, that it is the responsibility of gadget developer to
> > allocate buffers with proper alignment and size.
> 
> Document that please, I doubt this is documented anywhere, but it's
> clearly part of the API. Also, some checks might be put in place for
> the alignment , they might be in #ifdef DEBUG for all I care, but it
> would be nice to have such a check, since I'm worried someone will
> really be bitten.

I rely on the code embedded at cache_v7.c. It works and saved me a lot
of trouble (since it always print "ERROR" when buffer alignment and
size is wrong). 

Also I think, that those checks shall be done at invalidate_* and
flush_* cache routines, not at USB driver.

> 
> > > You might actually want to
> > > check for this condition and throw a warning in such a case.
> > 
> > The check is already implemented at ./arch/arm/cpu/armv7/cache_v7.c.
> 
> Yeah, for arm926ejs core as well. Maybe that check shall be shifted
> into the cache management routine prototypes somehow ... not all CPUs
> implement that check :-(

Maybe other ARM architectures shall have the cache management code
updated to work in a similar way to cache_v7.c ?

> 
> > It complains with "ERROR" message when start or end address is not
> > aligned (that is how I've discovered the unaligned buffers at UMS).
> 
> Yes.
> 
> > > I understand your argument with trying to not trash data, but then
> > > you will get a corruption during transfer, right ?
> > 
> > After applying those patches, the cache management would be
> > performed when the USB request is completed (in the UDC).
> > 
> > The only requirement for UDC is the correctly allocated buffer at
> > gadget.
> 
> Got it.

I will emphasize the need of correct buffer allocation at commit
message and add some comment to UDC in the v2.


-- 
Best regards,

Lukasz Majewski

Samsung R&D Institute Poland (SRPOL) | Linux Platform Group