linux-kernel - Re: [PATCH] fsldma: Support 40 bit DMA addresses where capable

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Message-Id: <fb0b5293-1cf3-4fcc-be9c-b5fe83f32325@app.fastmail.com>
Date: Tue, 22 Apr 2025 08:34:55 +0200
From: "Arnd Bergmann" <arnd@...db.de>
To: "Ben Collins" <bcollins@...nel.org>, dmaengine@...r.kernel.org
Cc: "Zhang Wei" <zw@...kernel.org>, "Vinod Koul" <vkoul@...nel.org>,
 linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fsldma: Support 40 bit DMA addresses where capable

On Tue, Apr 22, 2025, at 04:49, Ben Collins wrote:
> On 64-bit QorIQ platforms like T4240, the CPU supports 40-bit addressing
> and memory configurations > 64GiB. The fsldma driver is limiting itself
> to only 64GiB in all Elo configurations.
>
> Setup fsldma driver to make use of the full 40-bit addressing space,
> specifically on the e5500 and e6500 CPUs.

I don't think making the mask depend on a compile-time option is
correct, e.g. when you build a combined 32-bit kernel for e500 and
e5500, you set a different mask compared to an e500-only kernel.

The question here is whether the mask is a limitation of the
IP block or the bus it's connected to, of if there is any
limitation at all:

- The driver just writes the DMA address as a 64-bit register,
  so most likely the DMA device can in fact do wider addressing,
  and any limitation is either in the bus or the available
  memory

- SoCs that don't set a dma-ranges property in the parent bus
  are normally still capped to 32 bit DMA. I don't see those
  properties, so unless there is a special hack on those chips,
  you get 32 bit DMA regardless of what DMA mask the driver
  requests

- If there are chips that have more than 64GB of RAM installed
  but have a limitation in the way the DMA engine is wired
  up to 36 bits, that should be reflected in the dma-ranges
  property, not the device driver.

- If the limitation is indeed specific to the version of the
  IP block, this would normally need to be detected based on
  the compatible string of the DMA engine itself, not a compile
  time setting.

     Arnd