Re: [PATCH] dmaengine: bcm2835: Add slave dma support
From: Noralf Trønnes
Date: Thu Apr 16 2015 - 13:28:48 EST
Den 16.04.2015 08:30, skrev Rogier Wolff:
On Wed, Apr 15, 2015 at 08:53:07PM +0200, Noralf Trønnes wrote:
A 16-bit register can't hold a value of 65536.
Either the max value is 65535 or the register is 17-bits wide.
It is common for hardware registers to have the value "0" mean 65536
in case of a 16-bit register.
The hardware would then FIRST decrement the register and THEN check
for zero. This results in the behaviour that "1" requires one cycle to
complete, "10" requires ten cycles, and "0" means the same as the
total number of bitpatterns possible in the register. (256 for an
8-bit register, 65536 for a 16-bit register).
Another way to implement such a register in hardware would "check for
zero" first, and not do antyhing if the register equals zero. This
results in differnet behaviour for the "0" value.
That said: IMHO, the overhead of setting up 2 transfers for each 64k
block as opposed to only one results in such a small performance
penalty that I'd prefer to play it safe unless you're very sure you
can adequately test it. (Another option would be to set the maximum
transfer size to 0xf000: 60kbytes. Less than 10% extra transfers in
the long run than when aiming for the edge...)
Dom Cobley (Raspberry Pi) has just been in contact with
the hardware designer. He said:
65535 is the maximum transfer length of a LITE channel.
65536 will be treated as zero which is undefined
(it will actually do one transfer then stop)
Additional info from the datasheet about Lite channels:
The internal data structure is 128 bits instead of 256 bits.
This means that if you do a 128 bit wide read burst of more
than 1 beat, the DMA input register will be full and the read
bus will be stalled. The normal DMA engine can accept a read
burst of 2 without stalling. If you do a narrow 32 bit read
burst from the peripherals then the lite engine can cope with
a burst of 4 as opposed to a burst of 8 for the normal engine.
This suggest to me that we could go as far as the last 128-bit
boundary like this: (SZ_64K - 16)
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