[U-Boot] [PATCH v2] usb: dwc3: Allocate and flush dwc->ep0_trb in a cache aligned manner

[Marek Vasut]

On 10/17/2017 07:25 AM, Faiz Abbas wrote:
> 
> 
> On Monday 16 October 2017 08:52 PM, Marek Vasut wrote:
>> On 10/16/2017 04:51 PM, Felipe Balbi wrote:
>>>
>>> Hi,
>>>
>>> Faiz Abbas <faiz_abbas at ti.com> writes:
>>>>>>> Marek Vasut <marex at denx.de> writes:
>>>>>>>> On 10/16/2017 07:21 AM, Faiz Abbas wrote:
>>>>>>>>> A flush of the cache is required before any outbound DMA access can
>>>>>>>>> take place. The minimum size that can be flushed from the cache is
>>>>>>>>> one cache line size. Therefore, any buffer allocated for DMA should
>>>>>>>>> be in multiples of cache line size.
>>>>>>>>>
>>>>>>>>> Thus, allocate memory for ep0_trb in multiples of cache line size.
>>>>>>>>>
>>>>>>>>> Also, when local variable trb is assigned to dwc->ep0_trb[1] and used
>>>>>>>>> to flush cache, it leads to cache misaligned messages as only the base
>>>>>>>>> address dwc->ep0_trb is cache aligned.
>>>>>>>>>
>>>>>>>>> Therefore, flush cache using ep0_trb_addr which is always cache aligned.
>>>>>>>>>
>>>>>>>>> Signed-off-by: Faiz Abbas <faiz_abbas at ti.com>
>>>>>>>>
>>>>>>>> SGTM, Felipe, can you review this please ?
>>>>>>>
>>>>>>> is cache maintenance done correctly in u-boot? Isn't the whole idea of a
>>>>>>> coherent memory area that is is non-cacheable, non-bufferable memory?
>>>>>>>
>>>>>>> Also, why isn't the API itself guaranteeing alignment requirements?
>>>>>>>
>>>>>> There is no support in u-boot to make a memory area non-cacheable.
>>>>>> This is the definition of dma_alloc_coherent()
>>>>>>
>>>>>> static inline void *dma_alloc_coherent(size_t len, unsigned long *handle)
>>>>>> {
>>>>>>         *handle = (unsigned long)memalign(ARCH_DMA_MINALIGN, len);
>>>>>>         return (void *)*handle;
>>>>>> }
>>>>>>
>>>>>> This driver is mostly copied from kernel (where dma_alloc_coherent() is
>>>>>> what you describe) and extra flush_cache functions are added because of
>>>>>> U-Boot's inability to allocate coherent memory.
>>>>>
>>>>> then that's what should be fixed. No?
>>>>>
>>>>
>>>> You're right but that sounds like a long-term feature which will affect
>>>> a huge part of u-boot. Until it is implemented, I guess this is the best
>>>> way to handle the issue.
>>>
>>> Not my call to make. I'll defer to Marek and Tom
>>>
>> We're deep in RC anyway, so feel free to prepare a fix for next MW .
>>
> 
> Fix as in rebase same patch for next merge window?

As in, add support for marking memory area noncachable and then use it
here. It shouldn't be hard, it's only about some MMU table attributes.

-- 
Best regards,
Marek Vasut