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

[Felipe Balbi]

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

-- 
balbi