access efi variables

access efi variables

[Arend van Spriel]

I need to obtain a uefi variable so I went looking at the API in 
include/linux/efi.h. I found the following definition:

/*
  * The maximum size of VariableName + Data = 1024
  * Therefore, it's reasonable to save that much
  * space in each part of the structure,
  * and we use a page for reading/writing.
  */

struct efi_variable {
	efi_char16_t  VariableName[1024/sizeof(efi_char16_t)];
	efi_guid_t    VendorGuid;
	unsigned long DataSize;
	__u8          Data[1024];
	efi_status_t  Status;
	__u32         Attributes;
} __attribute__((packed));

But according to the specs the variable I need to obtain is 2k bytes. 
Should I expect trouble :-p

Regards,
Arend

Re: access efi variables

[Arend van Spriel]

On 09/13/2013 05:37 PM, Arend van Spriel wrote:
> I need to obtain a uefi variable so I went looking at the API in
> include/linux/efi.h. I found the following definition:
>
> /*
>   * The maximum size of VariableName + Data = 1024
>   * Therefore, it's reasonable to save that much
>   * space in each part of the structure,
>   * and we use a page for reading/writing.
>   */
>
> struct efi_variable {
>      efi_char16_t  VariableName[1024/sizeof(efi_char16_t)];
>      efi_guid_t    VendorGuid;
>      unsigned long DataSize;
>      __u8          Data[1024];
>      efi_status_t  Status;
>      __u32         Attributes;
> } __attribute__((packed));
>
> But according to the specs the variable I need to obtain is 2k bytes.

Maybe clarify that "the specs" is basically the feature information I 
received internally here.

> Should I expect trouble :-p
>
> Regards,
> Arend
>
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Re: access efi variables

[H. Peter Anvin]

On 09/13/2013 08:37 AM, Arend van Spriel wrote:
> I need to obtain a uefi variable so I went looking at the API in
> include/linux/efi.h. I found the following definition:
> 
> But according to the specs the variable I need to obtain is 2k bytes.
> Should I expect trouble :-p
> 

efivarfs doesn't have those limitations.

	-hpa

Re: access efi variables

[Arend van Spriel]

On 09/14/13 00:37, H. Peter Anvin wrote:
> On 09/13/2013 08:37 AM, Arend van Spriel wrote:
>> I need to obtain a uefi variable so I went looking at the API in
>> include/linux/efi.h. I found the following definition:
>>
>> But according to the specs the variable I need to obtain is 2k bytes.
>> Should I expect trouble :-p
>>
>
> efivarfs doesn't have those limitations.

Thanks, Peter

Looking into efivarfs it seems to use the functions that I looked at in 
efi.h so I guess I am misinterpreting the 1024 bytes limitation in the 
comment (either that or you are mistaken ;-) ). I need to access the 
variable from within a device driver so using efivarfs does not feel 
like the way to go here.

Regards,
Arend

Re: access efi variables

[Matt Fleming]

On Sat, 14 Sep, at 07:01:19PM, Arend van Spriel wrote:
> On 09/14/13 00:37, H. Peter Anvin wrote:
> >On 09/13/2013 08:37 AM, Arend van Spriel wrote:
> >>I need to obtain a uefi variable so I went looking at the API in
> >>include/linux/efi.h. I found the following definition:
> >>
> >>But according to the specs the variable I need to obtain is 2k bytes.
> >>Should I expect trouble :-p
> >>
> >
> >efivarfs doesn't have those limitations.
> 
> Thanks, Peter
> 
> Looking into efivarfs it seems to use the functions that I looked at
> in efi.h so I guess I am misinterpreting the 1024 bytes limitation
> in the comment (either that or you are mistaken ;-) ). I need to
> access the variable from within a device driver so using efivarfs
> does not feel like the way to go here.

Look at how efivarfs_file_read() is implemented, e.g. using
efivar_entry_get(). In particular, note how it kmalloc()'s a buffer for
reading the variable data into, thereby avoiding any 1024-byte
limitation.

-- 
Matt Fleming, Intel Open Source Technology Center