[PATCH v4] nvme/pci: Split 8-byte reads

[PATCH v4] nvme/pci: Split 8-byte reads

[Ard Biesheuvel]

The nvme pci driver had split 8-byte register reads using lo_hi_readq()
due to nvme controllers that do not support that sized access. This
behavior was inadvertently changed to readq(), which may break those
controllers. Restore the previous behavior.

Fixes: 7fd8930f26be4 ("nvme: add a common helper to read Identify Controller data")
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
---
v4: update comment and commit text according to Keith's guidance
v3: add comment explaining why we need the split read
v2: drop WARN_ON(), add reference to commit that [re-]introduced the issue
---
 drivers/nvme/host/pci.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/nvme/host/pci.c b/drivers/nvme/host/pci.c
index bb88681f4dc3..3fe0c5185646 100644
--- a/drivers/nvme/host/pci.c
+++ b/drivers/nvme/host/pci.c
@@ -2672,7 +2672,12 @@ static int nvme_pci_reg_write32(struct nvme_ctrl *ctrl, u32 off, u32 val)
 
 static int nvme_pci_reg_read64(struct nvme_ctrl *ctrl, u32 off, u64 *val)
 {
-	*val = readq(to_nvme_dev(ctrl)->bar + off);
+	/*
+	 * Split the 8-byte read into two 4-byte reads since all controllers
+	 * support 4 byte register reads, but some do not support the larger
+	 * access size.
+	 */
+	 *val = lo_hi_readq(to_nvme_dev(ctrl)->bar + off);
 	return 0;
 }
 
-- 
2.20.1


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Re: [PATCH v4] nvme/pci: Split 8-byte reads

[Keith Busch]

On Mon, Oct 07, 2019 at 08:22:44PM +0200, Ard Biesheuvel wrote:
> The nvme pci driver had split 8-byte register reads using lo_hi_readq()
> due to nvme controllers that do not support that sized access. This
> behavior was inadvertently changed to readq(), which may break those
> controllers. Restore the previous behavior.
> 
> Fixes: 7fd8930f26be4 ("nvme: add a common helper to read Identify Controller data")
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>

Looks good to me.

Reviewed-by: Keith Busch <kbusch@kernel.org>

> ---
>  drivers/nvme/host/pci.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/nvme/host/pci.c b/drivers/nvme/host/pci.c
> index bb88681f4dc3..3fe0c5185646 100644
> --- a/drivers/nvme/host/pci.c
> +++ b/drivers/nvme/host/pci.c
> @@ -2672,7 +2672,12 @@ static int nvme_pci_reg_write32(struct nvme_ctrl *ctrl, u32 off, u32 val)
>  
>  static int nvme_pci_reg_read64(struct nvme_ctrl *ctrl, u32 off, u64 *val)
>  {
> -	*val = readq(to_nvme_dev(ctrl)->bar + off);
> +	/*
> +	 * Split the 8-byte read into two 4-byte reads since all controllers
> +	 * support 4 byte register reads, but some do not support the larger
> +	 * access size.
> +	 */
> +	 *val = lo_hi_readq(to_nvme_dev(ctrl)->bar + off);
>  	return 0;
>  }
>  
> -- 
> 2.20.1
> 

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Re: [PATCH v4] nvme/pci: Split 8-byte reads

[Christoph Hellwig]

Looks good,

Reviewed-by: Christoph Hellwig <hch@lst.de>

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