Forcing CAP register value to make booting more reliable on the Acer Aspire Switch Alpha 12 tablet

Forcing CAP register value to make booting more reliable on the Acer Aspire Switch Alpha 12 tablet

[Sui Chen]

Hello, Linux-IDE mailing list,

I recently bought an Acer Aspire Switch Alpha 12 (Model Number: SA5-271) 
2-in-1 convertible computer. This computer has an Intel Skylake i5-6200U 
processor and a Lite-On CV1-8B256 SSD.

I noticed that the kernel will intermittently fail to detect the SSD as 
/dev/sda and may be fixed by changing seemingly unrelated settings in 
the BIOS (such as clearing secure boot databases) or with a "dirty hack" 
in libahci.c (tested on Kernel 4.8). When the SSD is not detected, the 
kernel will print an alert saying "Gave Up waiting for root device. 
Alert! /dev/disk/by-uuid/ does not exist. Dropping to a shell." Typing 
"blkid" in the initramfs shell shows no devices either.

When the SSD is detected the following lines can be seen in the dmesg 
output:
    [    1.347021] ahci 0000:00:17.0: version 3.0
    [    1.365569] ahci 0000:00:17.0: AHCI 0001.0301 32 slots 3 ports 6 
Gbps 0x7 impl SATA mode
    [    1.367519] ahci 0000:00:17.0: flags: 64bit ncq pm led clo only 
pio slum part deso sadm sds apst
    [    1.377574] scsi host0: ahci
    [    1.379465] scsi host1: ahci
    [    1.381373] scsi host2: ahci
    [    1.383060] ata1: SATA max UDMA/133 abar m2048@0xb1648000 port 
0xb1648100 irq 124
    [    1.384665] ata2: SATA max UDMA/133 abar m2048@0xb1648000 port 
0xb1648180 irq 124
    [    1.386305] ata3: SATA max UDMA/133 abar m2048@0xb1648000 port 
0xb1648200 irq 124

However, when the SSD is not detecting:
    [    1.337065] ahci 0000:00:17.0: version 3.0
-> [    1.343206] ahci 0000:00:17.0: implemented port map (0x7) contains 
more ports than nr_ports (2), using nr_ports
-> [    1.351165] ahci 0000:00:17.0: AHCI 0001.0301 32 slots 2 ports 6 
Gbps 0x0 impl SATA mode
    [    1.352323] ahci 0000:00:17.0: flags: 64bit ncq pm led clo only 
pio slum part deso sadm sds apst
    [    1.355960] scsi host0: ahci
    [    1.357292] scsi host1: ahci
-> [    1.358405] ata1: DUMMY
-> [    1.359466] ata2: DUMMY

One can note the differences in the marked lines in the dmesg output 
when the SSD is not detecting: 1) nr_ports becomes 2 instead of 3; 2) 
ATA1 and ATA2 are both DUMMY.


Adding the following lines in the function "ahci_save_initial_config" in 
file libahci.c, line 453 seems to fix this for now:
     if ((cap & 0xC734FF00) == 0xC734FF00) {
         dev_info(dev, "Forcing CAP to 0xC734FF02 and port_map to 0x7!\n");
         hpriv->saved_cap = cap = 0xC734FF02;
         hpriv->saved_port_map = port_map = 0x7;
     }

What the code does is to force port_map to become 0x7 and saved_cap to 
become 0xC734FF02. Actually, when the SSD is detecting the cap register 
holds a value of 0xC734FF02 but when it fails, cap can be either 
0xC734FF01 or 0xC734FF00.

My questions are:
Is this a bad way of solving this; can it possibly damage the computer?
Could this be possibly related to the BIOS of the specific computer?

Thank you so much!
2016-12-01
Sui