Hi Heiko,
On Wed, 22 May 2019 at 05:29, Heiko Stuebner heiko@sntech.de wrote:
Hi Simon,
Am Samstag, 18. Mai 2019, 18:08:58 CEST schrieb Simon Glass:
On Tue, 7 May 2019 at 09:59, Christoph Muellner christoph.muellner@theobroma-systems.com wrote:
From: Christoph Müllner christoph.muellner@theobroma-systems.com
Some machines have limited DMA engines, which cannot deal with arbitrary addresses. This patch introduces a function to model these restrictions on a machine level.
Signed-off-by: Christoph Müllner christoph.muellner@theobroma-systems.com Signed-off-by: Christoph Muellner christoph.muellner@theobroma-systems.com
Changes in v2: None
common/board_f.c | 5 +++++ include/init.h | 2 ++ 2 files changed, 7 insertions(+)
Can we handle this with driver model somehow? How does the kernel handle it?
The problem Christoph is trying to solve here is doing a mmc transfer from mmc to the sram (not main memory), which cannot use dma. So this exact problem doesn't happen in the kernel itself.
But back in veyron times we had a similar dma issue with anything accessing that area as dma hung the system, but the solution was just reserving the memblock: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?i...
As the commit describes, other options would've been soc-settings also going around the kernel's driver model or limiting the dma-able memory even more (to 2GB), so we opted to just reserve the upper 16MB completely.
OK I see.
Is there a device-tree binding for the DMA node that could provide this information.
I don't think so. At least in the kernel affecting the dma-mask is a setting for the using component (mmc, gmac, whatever), so would mean adapting every device doing dma ... and even then there wasn't a dt-binding for something like that, which in turn would've required to adapt every driver to set a specific per-soc dma-mask for Rockchip compatibles - hence the "simple" reserve above was the least intrusive option.
That's odd. Are you saying that some devices can DMA from SRAM and some cannot?
If the DMA is not allowed, could the DMA driver return -EPERM or similar when the attempt is made, and then the bounce buffer can be used if needed?
Also, where is this function called from?
In the theobroma u-boot it gets called from the bouncebuffer right now https://git.theobroma-systems.com/puma-u-boot.git/commit/?id=d68222d45b4e7f5... to check if the mmc drivers can dma directly or needs to use the bouncebuffer for reaching something like the sram.
OK ta.
Heiko
Regards, Simon