Re: [Xen-devel] [RFC 4/6] xen/console: Add support for early printk

[Julien Grall <julien.grall@xxxxxxxxxx>]

Hi Ian,

On 02/19/2014 11:19 AM, Ian Campbell wrote:
> On Sun, 2014-01-05 at 21:26 +0000, Julien Grall wrote:
>> On ARM, a function (early_printk) was introduced to output message when the
>> serial port is not initialized.
>>
>> This solution is fragile because the developper needs to know when the serial
>> port is initialized, to use either early_printk or printk. Moreover some
>> functions (mainly in common code), only use printk. This will result to a 
>> loss
>> of message sometimes.
>>
>> Directly call early_printk in console code when the serial port is not yet
>> initialized. For this purpose use serial_steal_fn.
> 
> This relies on nothing stealing the console over the period where the
> console is initialised. Perhaps that is already not advisable/possible?

serial_steal_fn is set in console_steal. This function checks if the
serial handle is valid.

This handle is only valid after console_init_preirq (which set
serial_steal_fn to NULL). So I think we are safe.

>>
>> Cc: Keir Fraser <keir@xxxxxxx>
>> Signed-off-by: Julien Grall <julien.grall@xxxxxxxxxx>
>> ---
>>  xen/drivers/char/console.c | 13 ++++++++++++-
>>  1 file changed, 12 insertions(+), 1 deletion(-)
>>
>> diff --git a/xen/drivers/char/console.c b/xen/drivers/char/console.c
>> index 532c426..f83c92e 100644
>> --- a/xen/drivers/char/console.c
>> +++ b/xen/drivers/char/console.c
>> @@ -28,6 +28,9 @@
>>  #include <asm/debugger.h>
>>  #include <asm/div64.h>
>>  #include <xen/hypercall.h> /* for do_console_io */
>> +#ifdef CONFIG_EARLY_PRINTK
>> +#include <asm/early_printk.h>
>> +#endif
>>  
>>  /* console: comma-separated list of console outputs. */
>>  static char __initdata opt_console[30] = OPT_CONSOLE_STR;
>> @@ -245,7 +248,12 @@ long read_console_ring(struct xen_sysctl_readconsole 
>> *op)
>>  static char serial_rx_ring[SERIAL_RX_SIZE];
>>  static unsigned int serial_rx_cons, serial_rx_prod;
>>  
>> -static void (*serial_steal_fn)(const char *);
>> +#ifndef CONFIG_EARLY_PRINTK
>> +static inline void early_puts(const char *str)
>> +{}
> 
> This duplicates bits of asm-arm/early_printk.h. I think if the feature
> is going to be used from common code then the common bits of the asm
> header should be moved to xen/early_printk.h. If any per-arch stuff
> remains then xen/e_p.h can include asm/e_p.h.

I will do.

Cheers,

-- 
Julien Grall

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