Re: [PATCH v3 07/15] ArmVirtPkg: Early serial port initialisation

[Julien Grall <julien@xen.org>]

Hi,

On 25/06/2020 12:42, Ard Biesheuvel wrote:
> (+ Julien)

Thank you for the CC!

> On 6/24/20 3:34 PM, Sami Mujawar wrote:
>> Initialise serial port early so that the platform
>> memory map can be logged.
>>
>> Signed-off-by: Sami Mujawar <sami.mujawar@arm.com>
>> ---
>>   ArmVirtPkg/PrePi/PrePi.c | 10 +++++-----
>>   1 file changed, 5 insertions(+), 5 deletions(-)
>>
>> diff --git a/ArmVirtPkg/PrePi/PrePi.c b/ArmVirtPkg/PrePi/PrePi.c
>> index 
>> 4f0c3f98bad63d0682cccd76a0a062d1fd4f46ab..5e144f4bdefb810708238097a3373f11d150bfe5 
>> 100755
>> --- a/ArmVirtPkg/PrePi/PrePi.c
>> +++ b/ArmVirtPkg/PrePi/PrePi.c
>> @@ -1,6 +1,6 @@
>>   /** @file
>>   *
>> -*  Copyright (c) 2011-2014, ARM Limited. All rights reserved.
>> +*  Copyright (c) 2011-2020, ARM Limited. All rights reserved.
>>   *
>>   *  SPDX-License-Identifier: BSD-2-Clause-Patent
>>   *
>> @@ -60,16 +60,16 @@ PrePiMain (
>>     //
>>     InvalidateDataCacheRange((VOID *)(UINTN)PcdGet64 
>> (PcdFdBaseAddress), PcdGet32 (PcdFdSize));
>> -  // Initialize MMU and Memory HOBs (Resource Descriptor HOBs)
>> -  Status = MemoryPeim (UefiMemoryBase, FixedPcdGet32 
>> (PcdSystemMemoryUefiRegionSize));
>> -  ASSERT_EFI_ERROR (Status);
>> -
>>     // Initialize the Serial Port
>>     SerialPortInitialize ();
>>     CharCount = AsciiSPrint (Buffer,sizeof (Buffer),"UEFI firmware 
>> (version %s built at %a on %a)\n\r",
>>       (CHAR16*)PcdGetPtr(PcdFirmwareVersionString), __TIME__, __DATE__);
>>     SerialPortWrite ((UINT8 *) Buffer, CharCount);
>> +  // Initialize MMU and Memory HOBs (Resource Descriptor HOBs)
>> +  Status = MemoryPeim (UefiMemoryBase, FixedPcdGet32 
>> (PcdSystemMemoryUefiRegionSize));
>> +  ASSERT_EFI_ERROR (Status);
>> +
>>     // Create the Stacks HOB (reserve the memory for all stacks)
>>     StacksSize = PcdGet32 (PcdCPUCorePrimaryStackSize);
>>     BuildStackHob (StacksBase, StacksSize);
>>
> 
> We need to check whether this works on Xen - I don't remember whether we 
> use some kind of cache coherent shared memory ring there for the serial 
> console that only works if the MMU and caches are enabled. Julien?

Xen Console is using a shared memory ring that requires to be cache 
coherent. So you wouldn't be able to use the current driver before the 
MMU and caches are enabled.

It might be possible to make it work by issuing cache maintenance when 
sending the characters.

Cheers,

-- 
Julien Grall