linux-kernel - Re: [PATCH 2/5] staging: vc04_services: Remove cache-line-size property.

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Date:   Tue, 06 Mar 2018 11:02:04 -0800
From:   Eric Anholt <eric@...olt.net>
To:     Stefan Wahren <stefan.wahren@...e.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Mark Rutland <mark.rutland@....com>,
        Rob Herring <robh+dt@...nel.org>, devicetree@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Phil Elwell <phil@...pberrypi.org>
Cc:     linux-kernel@...r.kernel.org,
        bcm-kernel-feedback-list@...adcom.com,
        linux-rpi-kernel@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 2/5] staging: vc04_services: Remove cache-line-size property.

Stefan Wahren <stefan.wahren@...e.com> writes:

> Hi Eric,
>
>
> Am 05.03.2018 um 21:28 schrieb Eric Anholt:
>> This was just a way for the DT-passed value to get out of sync with
>> what Linux has configured the ARM for.
>>
>> Signed-off-by: Eric Anholt <eric@...olt.net>
>> ---
>>   .../interface/vchiq_arm/vchiq_2835_arm.c           | 25 +++++++---------------
>>   .../interface/vchiq_arm/vchiq_pagelist.h           |  1 -
>>   2 files changed, 8 insertions(+), 18 deletions(-)
>>
>> diff --git a/drivers/staging/vc04_services/interface/vchiq_arm/vchiq_2835_arm.c b/drivers/staging/vc04_services/interface/vchiq_arm/vchiq_2835_arm.c
>> index b59ef14890aa..e0e01c812036 100644
>> --- a/drivers/staging/vc04_services/interface/vchiq_arm/vchiq_2835_arm.c
>> +++ b/drivers/staging/vc04_services/interface/vchiq_arm/vchiq_2835_arm.c
>> @@ -77,7 +77,6 @@ struct vchiq_pagelist_info {
>>   };
>>   
>>   static void __iomem *g_regs;
>> -static unsigned int g_cache_line_size = sizeof(CACHE_LINE_SIZE);
>>   static unsigned int g_fragments_size;
>>   static char *g_fragments_base;
>>   static char *g_free_fragments;
>> @@ -117,15 +116,7 @@ int vchiq_platform_init(struct platform_device *pdev, VCHIQ_STATE_T *state)
>>   	if (err < 0)
>>   		return err;
>>   
>> -	err = of_property_read_u32(dev->of_node, "cache-line-size",
>> -				   &g_cache_line_size);
>> -
>> -	if (err) {
>> -		dev_err(dev, "Missing cache-line-size property\n");
>> -		return -ENODEV;
>> -	}
>> -
>> -	g_fragments_size = 2 * g_cache_line_size;
>> +	g_fragments_size = 2 * cache_line_size();
>>   
>>   	/* Allocate space for the channels in coherent memory */
>>   	slot_mem_size = PAGE_ALIGN(TOTAL_SLOTS * VCHIQ_SLOT_SIZE);
>> @@ -548,9 +539,9 @@ create_pagelist(char __user *buf, size_t count, unsigned short type)
>>   
>>   	/* Partial cache lines (fragments) require special measures */
>>   	if ((type == PAGELIST_READ) &&
>> -		((pagelist->offset & (g_cache_line_size - 1)) ||
>> +		((pagelist->offset & (cache_line_size() - 1)) ||
>>   		((pagelist->offset + pagelist->length) &
>> -		(g_cache_line_size - 1)))) {
>> +		 (cache_line_size() - 1)))) {
>>   		char *fragments;
>>   
>>   		if (down_interruptible(&g_free_fragments_sema) != 0) {
>> @@ -598,10 +589,10 @@ free_pagelist(struct vchiq_pagelist_info *pagelistinfo,
>>   			g_fragments_size;
>>   		int head_bytes, tail_bytes;
>>   
>> -		head_bytes = (g_cache_line_size - pagelist->offset) &
>> -			(g_cache_line_size - 1);
>> +		head_bytes = (cache_line_size() - pagelist->offset) &
>> +			(cache_line_size() - 1);
>>   		tail_bytes = (pagelist->offset + actual) &
>> -			(g_cache_line_size - 1);
>> +			(cache_line_size() - 1);
>
> should it be so easy? Back in 2016 we said that cache_line_size() won't 
> work. I always thought that we need the cache line size of L2 not of the 
> L1 one.
>
> Did you already test the behavior for these combinations?
> BCM2835 ARM32, expected cache line size = 32
> BCM2836 ARM32, expected cache line size = 64
> BCM2837 ARM32, expected cache line size = 64
> BCM2837 ARM64, expected cache line size = 64

I didn't explicitly test, but was going by:

config ARM_L1_CACHE_SHIFT_6
	bool
	default y if CPU_V7
	help
	  Setting ARM L1 cache line size to 64 Bytes.

config ARM_L1_CACHE_SHIFT_7
	bool
	help
	  Setting ARM L1 cache line size to 128 Bytes.

config ARM_L1_CACHE_SHIFT
	int
	default 7 if ARM_L1_CACHE_SHIFT_7
	default 6 if ARM_L1_CACHE_SHIFT_6
	default 5

and only L1_CACHE_SHIFT_7 gets selected by UNIPHIER and neither one is
accessible by menus.

I think you're technically correct that it's the size of L2 that matters
(or, specifically, the hardcoded value that the firmware is using on its
side for the fragments list handling.  It overrides a cache-line-size DT
property with that number if present).  However, I think L1==L2 cache
line size this should be a safe assumption for us.

Phil, any opinion?

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