[PATCH 4/6] imx8m: Restrict usable memory to space below 4G boundary

[Marek Vasut]

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.schrempf at 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.fan at 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.