linux-kernel - Re: [PATCH] cxl: Replace HDM decoder granularity magic numbers

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Date:   Mon, 22 Aug 2022 10:17:03 -0700
From:   Davidlohr Bueso <dave@...olabs.net>
To:     Adam Manzanares <a.manzanares@...sung.com>
Cc:     "alison.schofield@...el.com" <alison.schofield@...el.com>,
        "vishal.l.verma@...el.com" <vishal.l.verma@...el.com>,
        "ira.weiny@...el.com" <ira.weiny@...el.com>,
        "widawsk@...nel.org" <widawsk@...nel.org>,
        "dan.j.williams@...el.com" <dan.j.williams@...el.com>,
        "Jonathan.Cameron@...wei.com" <Jonathan.Cameron@...wei.com>,
        "linux-cxl@...r.kernel.org" <linux-cxl@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] cxl: Replace HDM decoder granularity magic numbers

On Mon, 22 Aug 2022, Adam Manzanares wrote:

>When reviewing the CFMWS parsing code that deals with the HDM decoders,
>I noticed a couple of magic numbers. This commit replaces these magic numbers
>with constants defined by the CXL 2.0 specification.

Please use 3.0 spec :)

Actually the whole drivers/cxl/* could use updating the comments for 3.0.

>Signed-off-by: Adam Manzanares <a.manzanares@...sung.com>
>---
> drivers/cxl/cxl.h | 11 +++++++----
> 1 file changed, 7 insertions(+), 4 deletions(-)
>
>diff --git a/drivers/cxl/cxl.h b/drivers/cxl/cxl.h
>index f680450f0b16..ba3a66b5b9cd 100644
>--- a/drivers/cxl/cxl.h
>+++ b/drivers/cxl/cxl.h
>@@ -61,6 +61,10 @@
> #define CXL_HDM_DECODER0_SKIP_LOW(i) CXL_HDM_DECODER0_TL_LOW(i)
> #define CXL_HDM_DECODER0_SKIP_HIGH(i) CXL_HDM_DECODER0_TL_HIGH(i)
>
>+/* HDM decoder control register constants CXL 2.0 8.2.5.12.7 */

This now becomes 8.2.4.19.7.

>+#define CXL_DECODER_MIN_GRANULARITY 256
>+#define CXL_DECODER_MAX_GRANULARITY_ORDER 6

Considering there is a single user, I don't think we need to add
the CXL_DECODER_MAX_GRANULARITY_ORDER. And if a new one is added
it would be better to keep symmetry (bytes) and just do
CXL_DECODER_MAX_GRANULARITY, but that's probably the reason why
it doesn't exist in the first place.

>+
> static inline int cxl_hdm_decoder_count(u32 cap_hdr)
> {
>	int val = FIELD_GET(CXL_HDM_DECODER_COUNT_MASK, cap_hdr);
>@@ -71,9 +75,9 @@ static inline int cxl_hdm_decoder_count(u32 cap_hdr)
> /* Encode defined in CXL 2.0 8.2.5.12.7 HDM Decoder Control Register */
> static inline int cxl_to_granularity(u16 ig, unsigned int *val)
> {
>-	if (ig > 6)
>+	if (ig > CXL_DECODER_MAX_GRANULARITY_ORDER)
>		return -EINVAL;
>-	*val = 256 << ig;
>+	*val = CXL_DECODER_MIN_GRANULARITY << ig;
>	return 0;
> }
>
>@@ -96,7 +100,7 @@ static inline int cxl_to_ways(u8 eniw, unsigned int *val)
>
> static inline int granularity_to_cxl(int g, u16 *ig)
> {
>-	if (g > SZ_16K || g < 256 || !is_power_of_2(g))
>+	if (g > SZ_16K || g < CXL_DECODER_MIN_GRANULARITY || !is_power_of_2(g))

Looks good.

Thanks,
Davidlohr