[U-Boot] [PATCH] usb: dwc3: gadget: make cache-maintenance on event buffers more robust

[Dr. Philipp Tomsich]

> On 04 Apr 2017, at 22:09, Marek Vasut <marex at denx.de> wrote:
> 
>> The DWC3 flush expands to a clean+invalidate. It is not wrong, as long as
>> it is used as in my patch:
>> a. before the first time data is expected to be written by the peripheral (i.e.
>> before the peripheral is started)—to ensure that the cache line is not cached
>> any longer…
> 
> So invalidate() is enough ?

If I had to write this from scratch, I’d got with the paranoid sequence of:

	handler():
	{
		invalidate
		do my stuff
		clean
	}

However, some architectures in U-Boot (e.g. ARMv8) don’t implement the
invalidate verb. Given this, I’d rather stay as close to what’s already there.

Note that using flush (i.e. clean+invalidate) aligns with how caches are
managed throughout various other drivers in U-Boot.

> 
>> b. after the driver modifies any buffers (i.e. anything modified will be written
>> back) and before it next reads the buffers expecting possibly changed data
>> (i.e. invalidating).
> 
> So flush+invalidate ? Keep in mind this driver may not be used on
> ARMv7/v8 only …

Yes, a clean+invalidate.
The flush_dcache_range(…, …) function in U-Boot implements C+I semantics
at least on arm, arm64, avr32, powerpc, xtensa …

Regards,
Philipp.