Re: [PATCH v2] riscv: mm: fix SWIOTLB initialization for systems with DRAM above 4GB

[Paul Walmsley <pjw@kernel.org>]

Hi Troy, Anirudh,

a few questions - 

On Tue, 14 Jul 2026, Anirudh Srinivasan wrote:

> On Wed, Apr 29, 2026 at 07:41:54PM +0800, Troy Mitchell wrote:
> > On RISC-V platforms where the entire physical memory (DRAM) resides
> > above the 32-bit address space (i.e., above dma32_phys_limit), the
> > current SWIOTLB initialization logic fails.

Is this primarily for the SpacemiT K3?  Or are there other 
platforms that this fixes?

> > This patch addresses two interconnected issues on such platforms:
> > 
> > 1. Incorrect 32-bit DMA bounce assumption:
> > The existing condition `max_pfn > PFN_DOWN(dma32_phys_limit)` assumes
> > that a 32-bit DMA bounce buffer is required simply because the maximum
> > PFN exceeds the 32-bit limit. However, if all DRAM starts above 4GB,
> > no memory exists below the limit to satisfy this allocation. Fix
> > this by adding a check to ensure `memblock_start_of_DRAM()` is actually
> > below the 32-bit limit before enforcing 32-bit SWIOTLB.
> > 
> > 2. kmalloc() bounce buffer allocation failure on non-coherent systems:
> > For non-coherent hardware, a bounce buffer is still mandatory for
> > cache-line-aligned kmalloc(), even if 32-bit DMA bouncing is skipped.

I'm not sure I'm parsing this correctly.  Shouldn't this read "for 
*non*-cache-aligned kmalloc()" ?  Or am I missing the point?


- Paul

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