Hi again,
Russell King - ARM Linux wrote:
On Mon, Dec 05, 2011 at 06:35:56PM +0100, Ulf Hansson wrote:
Instead of reading a register value everytime we need to
apply a new value for it, maintain a cached copy for it.
This also means we are able to skip writes that are not
needed.
I'm not sure this is a good idea. The ARM Primecells require a certain
number of bus clocks and MCLK periods between writes to both these
registers, and reading them back helps to ensure that we conform to
that requirement. Maintaining a cached copy of them allows faster
writes to these registers which could cause that requirement to be
violated.
I were just about to update my patch according to your proposal, when I
realized the only place were the registers were previously "read before
write", were at an SDIO corner case in pio_write.
Earlier the register values were always written, without considering the
old value. Thus the impact with this patch is kind of only decreasing
the number of writes and affects a corner case for SDIO.
Do you anyway prefer to add a register "read before write" even if it
never has been needed before?
What you could do is read the register, modify, and check whether the
modification has had any effect before writing it back. That will
allow unnecessary writes to still be skipped.
Sorry for spamming you.
BR
Ulf Hansson
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