On 6/15/21 2:28 AM, Peng Fan (OSS) wrote:


On 2021/6/7 20:38, Marek Vasut wrote:
On 6/7/21 2:05 PM, Frieder Schrempf wrote:
From: Frieder Schrempf <frieder.schre...@kontron.de>

Some IPs have their accessible address space restricted by the
interconnect. Let's make sure U-Boot only ever uses the space below
the 4G address boundary (which is 3GiB big), even when the effective
available memory is bigger.

We implement board_get_usable_ram_top() for all i.MX8M SoCs, as the
whole family is affected by this.

Shouldn't only those specific IP drivers handle buffers in the 64bit space somehow ? E.g. using a bounce buffer ?

That could cause extra mem copy.

If you want to avoid the extra memcopy, then make sure the buffers are not allocated above 4 GiB boundary. Then the bounce buffer does no copy.

This board_get_usable_ram_top() is just a temporary workaround for platforms with broken drivers which are not fixed yet, so please fix the drivers instead.

Bounce buffer would be good for systems
that take U-Boot as UEFI firmware, because U-Boot would be located at
high end, but in the middle just top of 4GB.

The bounce buffer is a necessity for IPs which cannot address more than the 4 GiB of memory. In fact, it would be even better to handle DT dma-ranges, but that is the next step.

I not object this patch, but it also be good if bounce buffer be added
for improvement.

Reviewed-by: Peng Fan <peng....@nxp.com>

I do object to this, since it increases the proliferation of this broken board_get_usable_ram_top() workaround instead of fixing the drivers properly.

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