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

[Serge Semin <fancer.lancer@gmail.com>]

On Sat, Sep 10, 2022 at 03:35:45PM +0300, Serge Semin wrote:
> 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
  ^
  `-- I meant fragmentation of course...

> 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?