linux-kernel - Re: [PATCH V4 1/4] efi: x86: rearrange efi_mem

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Date:	Mon, 15 Jun 2015 15:24:46 -0700
From:	"Zhang, Jonathan Zhixiong" <zjzhang@...eaurora.org>
To:	Matt Fleming <matt@...eblueprint.co.uk>
CC:	Matt Fleming <matt.fleming@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	ying.huang@...el.com, fu.wei@...aro.org, al.stone@...aro.org,
	bp@...en8.de, linux-efi@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH V4 1/4] efi: x86: rearrange efi_mem_attributes()


On 6/15/2015 10:09 AM, Matt Fleming wrote:
> On Fri, 12 Jun, at 06:01:44PM, Jonathan (Zhixiong) Zhang wrote:
>> From: "Jonathan (Zhixiong) Zhang" <zjzhang@...eaurora.org>
>>
>> x86 and ia64 implement efi_mem_attributes() differently. This
>> function needs to be available for other arch (such as arm64)
>> as well, such as for the purpose of ACPI/APEI.
>>
>> ia64 efi does not setup memmap variable and does not set
>> EFI_MEMMAP flag, so it needs to have its unique implementation
>> of efi_mem_attributes().
>
> Something like the above paragraph...
>
>> @@ -517,3 +517,21 @@ char * __init efi_md_typeattr_format(char *buf, size_t size,
>>   			 attr & EFI_MEMORY_UC      ? "UC"  : "");
>>   	return buf;
>>   }
>
> needs to go here, i.e. as documentation for efi_mem_atributes(). You
> need to explain why this function is marked as __weak at the function
> definition site.
>
>> +u64 __weak efi_mem_attributes(unsigned long phys_addr)
>> +{
>> +	efi_memory_desc_t *md;
>> +	void *p;
>> +
>> +	if (!efi_enabled(EFI_MEMMAP))
>> +		return 0;
>> +
>> +	for (p = memmap.map; p < memmap.map_end; p += memmap.desc_size) {
>> +		md = p;
>> +		if ((md->phys_addr <= phys_addr) &&
>> +		    (phys_addr < (md->phys_addr +
>> +		    (md->num_pages << EFI_PAGE_SHIFT))))
>> +			return md->attribute;
>> +	}
>> +	return 0;
>> +}
>
> because otherwise people are going to read this in the future and think,
>
>   "efi_mem_attribute() doesn't quite work how I want it to for my arch,
>   but it's __weak, so I'll override it"
>
> which is absolutely not the practice we should be promoting.
>
> How about something like this,
>
> 	/*
> 	 * efi_mem_attributes - lookup memmap attributes for physical address
> 	 * @phys_addr: the physical address to lookup
> 	 *
> 	 * Search in the EFI memory map for the region covering
> 	 * @phys_addr. Returns the EFI memory attributes if the region
> 	 * was found in the memory map, 0 otherwise.
> 	 *
> 	 * Despite being marked __weak most architectures should *not*
> 	 * override this function. It is __weak solely for the benefit
> 	 * of ia64 which has a funky EFI memory map that doesn't work
> 	 * the same way as other architectures.
> 	 */
>
Thank you Matt for the suggestion. Will do so accordingly.

-- 
Jonathan (Zhixiong) Zhang
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
a Linux Foundation Collaborative Project
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