Re: [PATCH 6/6] bmips: dma: drop redundant boot_cpu_type in arch_dma_sync

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On 5/3/24 12:39, Christian Marangi wrote:
On Fri, May 03, 2024 at 12:07:45PM -0700, Florian Fainelli wrote:
On 5/3/24 06:54, Christian Marangi wrote:
Drop redundant boot_cpu_type in arch_sync_dma_for_cpu_all. These needs
to be parsed only once and we can make use of bmips_rac_flush_disable to
disable RAC flush on unsupported CPU.

Set this value in bmips_cpu_setup for unsupported CPU to skip this
redundant check every time DMA needs to be synced.

Signed-off-by: Christian Marangi <ansuelsmth@xxxxxxxxx>

You are taking a shortcut that is reasonable in premise, but keying off the
bmips_rac_flush_disable is IMHO misleading. The RAC is enabled in the
BMIPS5000 and BMIPS5200 cores, just it does not need SW management unlike
earlier cores.

If you renamed it to bmips_rac_flush_needed that might be more compelling.
Also, the other reason is that on a kernel that was configured for
supporting only BMIPS5000 and BMIPS5200 CPUs, I think we could get some
decent dead code elimination of the boot_cpu_type() check, which would not
be the case.

I was a bit confused by the last part, should I drop this or just rename
the variable? Cause I think for kernel that support ONLY those CPU I
guess the DMA function will be optimized anyway since the bool will
always be false I guess?

I don't think it can be optimized, I would drop that patch. This is a hot-path and so any optimization is welcome.
--
Florian

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