linux-kernel - Re: [PATCH] drm/radeon: bypass no_64bit

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Message-Id: <e86b823f-5e83-4105-8e4d-1db141d088a4@app.fastmail.com>
Date: Mon, 22 Dec 2025 22:32:43 +0100
From: "Arnd Bergmann" <arnd@...db.de>
To: "Han Gao" <gaohan@...as.ac.cn>,
 "Alex Deucher" <alexander.deucher@....com>,
 Christian König <christian.koenig@....com>,
 "Dave Airlie" <airlied@...il.com>, "Simona Vetter" <simona@...ll.ch>,
 amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
 linux-kernel@...r.kernel.org
Cc: "Han Gao" <rabenda.cn@...il.com>, linux-riscv@...ts.infradead.org,
 sophgo@...ts.linux.dev
Subject: Re: [PATCH] drm/radeon: bypass no_64bit_msi with new msi64 parameter

On Sat, Dec 20, 2025, at 17:33, Han Gao wrote:
> diff --git a/drivers/gpu/drm/radeon/radeon_drv.c 
> b/drivers/gpu/drm/radeon/radeon_drv.c
> index 87fd6255c114..53af28494c03 100644
> --- a/drivers/gpu/drm/radeon/radeon_drv.c
> +++ b/drivers/gpu/drm/radeon/radeon_drv.c
> @@ -249,6 +249,10 @@ int radeon_cik_support = -1;
>  MODULE_PARM_DESC(cik_support, "CIK support (1 = enabled, 0 = disabled, 
> -1 = default)");
>  module_param_named(cik_support, radeon_cik_support, int, 0444);
> 
> +int radeon_msi64;
> +MODULE_PARM_DESC(msi64, "MSI64 support (1 = enabled, 0 = disabled)");
> +module_param_named(msi64, radeon_msi64, int, 0444);
> +

As with the hda-intel patch, this should not be a module argument,
but we should have the kernel figure out what to do itself.

> diff --git a/drivers/gpu/drm/radeon/radeon_irq_kms.c 
> b/drivers/gpu/drm/radeon/radeon_irq_kms.c
> index 9961251b44ba..62eb5a6968ff 100644
> --- a/drivers/gpu/drm/radeon/radeon_irq_kms.c
> +++ b/drivers/gpu/drm/radeon/radeon_irq_kms.c
> @@ -250,7 +250,7 @@ static bool radeon_msi_ok(struct radeon_device 
> *rdev)
>  	 * of address for "64-bit" MSIs which breaks on some platforms, 
> notably
>  	 * IBM POWER servers, so we limit them
>  	 */
> -	if (rdev->family < CHIP_BONAIRE) {
> +	if (rdev->family < CHIP_BONAIRE && !radeon_msi64) {
>  		dev_info(rdev->dev, "radeon: MSI limited to 32-bit\n");
>  		rdev->pdev->no_64bit_msi = 1;

According to the comment above it, the device can apparently
do 40-bit addressing but not use the entire 64-bit space.

I assume the SG2042 chip has the irqchip somewhere above the
32-bit line but below the 40-bit line, so it ends up working.

I wonder if the msi_verify_entries() function should check
against dev->coherent_dma_mask instead of checking the
upper 32 bits for being nonzero, that probably gives you
the desired behavior.

     Arnd