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

[Faiz Abbas]

Hi,

On Monday 16 October 2017 07:50 PM, Felipe Balbi wrote:
> 
> Hi,
> 
> Faiz Abbas <faiz_abbas at ti.com> writes:
>> Hi Felipe,
>>
>> On Monday 16 October 2017 07:25 PM, Felipe Balbi wrote:
>>>
>>> Hi,
>>>
>>> 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.

Thanks,
Faiz