On 05/31/2017 05:27 AM, Lu Baolu wrote: > Hi, > > On 05/30/2017 09:46 PM, Vlastimil Babka wrote: >> On 03/21/2017 09:01 AM, Lu Baolu wrote: >>> XHCI debug capability (DbC) is an optional but standalone >>> functionality provided by an xHCI host controller. Software >>> learns this capability by walking through the extended >>> capability list of the host. XHCI specification describes >>> DbC in the section 7.6. >>> >>> This patch introduces the code to probe and initialize the >>> debug capability hardware during early boot. With hardware >>> initialized, the debug target (system on which this code is >>> running) will present a debug device through the debug port >>> (normally the first USB3 port). The debug device is fully >>> compliant with the USB framework and provides the equivalent >>> of a very high performance (USB3) full-duplex serial link >>> between the debug host and target. The DbC functionality is >>> independent of the xHCI host. There isn't any precondition >>> from the xHCI host side for the DbC to work. >>> >>> One use for this feature is kernel debugging, for example >>> when your machine crashes very early before the regular >>> console code is initialized. Other uses include simpler, >>> lockless logging instead of a full-blown printk console >>> driver and klogd. >>> >>> Cc: Ingo Molnar <mingo@xxxxxxxxxx> >>> Cc: Mathias Nyman <mathias.nyman@xxxxxxxxxxxxxxx> >>> Signed-off-by: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx> >> ... >> >>> + >>> +#define XDBC_TRACE >>> +#ifdef XDBC_TRACE >>> +#define xdbc_trace trace_printk >> Did you forget to remove the #define XDBC_TRACE? >> >> Enabling this driver brings the "trace_printk() being used. Allocating >> extra memory. This means that this is a DEBUG kernel and it is unsafe >> for production use." message in 4.12-rcX dmesg. > > This feature is only for a DEBUG kernel, should not be enabled for > any production kernel. This was the reason I enabled trace_printk() > by default. Hmm, but it seems we generally enable all these early printk features/drivers in our distro kernels. They are not active without a early_printk=X bootparam anyway, right? It's much more convenient to e.g. just tell customer to change a param when debugging something than to install a debug kernel. So I wouldn't agree that only a DEBUG kernel should have this compiled in. Thanks, Vlastimil > Best regards, > Lu Baolu > >> Thanks, >> Vlastimil >> > -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
