[PATCH] swiotlb: set IO TLB segment size via cmdline

[Jan Beulich]

On 14.09.2021 17:10, Roman Skakun wrote:
> From: Roman Skakun <roman_skakun at epam.com>
> 
> It is possible when default IO TLB size is not
> enough to fit a long buffers as described here [1].
> 
> This patch makes a way to set this parameter
> using cmdline instead of recompiling a kernel.
> 
> [1] https://www.xilinx.com/support/answers/72694.html

I'm not convinced the swiotlb use describe there falls under "intended
use" - mapping a 1280x720 framebuffer in a single chunk? (As an aside,
the bottom of this page is also confusing, as following "Then we can
confirm the modified swiotlb size in the boot log:" there is a log
fragment showing the same original size of 64Mb.

> --- a/arch/mips/cavium-octeon/dma-octeon.c
> +++ b/arch/mips/cavium-octeon/dma-octeon.c
> @@ -237,7 +237,7 @@ void __init plat_swiotlb_setup(void)
>  		swiotlbsize = 64 * (1<<20);
>  #endif
>  	swiotlb_nslabs = swiotlbsize >> IO_TLB_SHIFT;
> -	swiotlb_nslabs = ALIGN(swiotlb_nslabs, IO_TLB_SEGSIZE);
> +	swiotlb_nslabs = ALIGN(swiotlb_nslabs, swiotlb_io_seg_size());

In order to be sure to catch all uses like this one (including ones
which make it upstream in parallel to yours), I think you will want
to rename the original IO_TLB_SEGSIZE to e.g. IO_TLB_DEFAULT_SEGSIZE.

> @@ -81,15 +86,30 @@ static unsigned int max_segment;
>  static unsigned long default_nslabs = IO_TLB_DEFAULT_SIZE >> IO_TLB_SHIFT;
>  
>  static int __init
> -setup_io_tlb_npages(char *str)
> +setup_io_tlb_params(char *str)
>  {
> +	unsigned long tmp;
> +
>  	if (isdigit(*str)) {
> -		/* avoid tail segment of size < IO_TLB_SEGSIZE */
> -		default_nslabs =
> -			ALIGN(simple_strtoul(str, &str, 0), IO_TLB_SEGSIZE);
> +		default_nslabs = simple_strtoul(str, &str, 0);
>  	}
>  	if (*str == ',')
>  		++str;
> +
> +	/* get max IO TLB segment size */
> +	if (isdigit(*str)) {
> +		tmp = simple_strtoul(str, &str, 0);
> +		if (tmp)
> +			io_tlb_seg_size = ALIGN(tmp, IO_TLB_SEGSIZE);

>From all I can tell io_tlb_seg_size wants to be a power of 2. Merely
aligning to a multiple of IO_TLB_SEGSIZE isn't going to be enough.

Jan