Re: [PATCH 1/2] nvme-hwmon: Cache-line-align the NVME SMART log-buffer

[Serge Semin]

On Sat, Sep 10, 2022 at 07:30:45AM +0200, Christoph Hellwig wrote:
> I think this will work, but unless we have to I'd generally prefer
> to just split dta that is DMAed into into a separate allocation.
> That is, do a separate kmalloc for the nvme_smart_log structure.

Well, both approaches will solve the denoted problem. I am just
wondering why do you think that the kmalloc-ed buffer is more
preferable? IMO it is a bit less suitable since increases the memory
granularity - two kmalloc's instead of one. Moreover it makes the code
a bit more complex for the same reason of having two mallocs and two
frees. Meanwhile using the ____cacheline_aligned qualifier to prevent
the noncoherent DMA problem is a standard approach.

What would be the best solution if we had a qualifier like this:
#ifdef CONFIG_DMA_NONCOHERENT
#define ____dma_buffer ____cacheline_aligned
#else
#define ____dma_buffer
#endif
and used it instead of the direct ____cacheline_aligned utilization.

-Sergey

> 
> Guenter, is this ok with you?